The relationship between c'est and une pizza is almost as if the verb est does not exist. Imagine it as Ce = a pizza. This = a pizza. That = a pizza. It = a pizza. Grammatically, it is a copular relationship. Some languages have different verbs to express this different aspect of to be. The difference is between " He is a man" and "He is rich"The first expresses a permanent characteristic, the second a temporary state. English uses the same verb, French uses c'est instead of il est (C'est un homme et il est riche), Irish has two different verbs. (Is fear é agus tá sé saibhir) and Spanish goes with "Soy un hombre" and "Estoy un hombre rico". The Spanish is a bit more nuanced. Estoy rico = I am rich (at present), but soy rico would = I am rich (as part of my essence, I am intrinsically rich: it's who I am).
Basically, the rule is that "il/elle est" and "ils/elles sont" change to "c'est" and "ce sont" before a modified noun, that is, a noun preceded by a modifier. A modifier can be:
- an article: un, une, des, le, la, l', les
- a number: un, deux...
- a possessive adjective: mon, ton, son, ma, ta, sa, notre, votre, leur, mes, tes, ses, nos, vos, leurs
- a demonstrative adjective: ce, cet, cette, ces
In this sentence, "pizza" is preceded by the article "une", so it is a modified noun => use "c'est".
These articles go into more detail and are worth a read.
Also, the Tips and Notes for the Gallicism skill (click the lightbulb icon when you open the skill) give more information.
English nouns do not really have gender any more. There is no difference in treatment between a/ the man and a/the woman. Gender still exists with pronoun forms, so you need he/she/it. But natural gender applies. A man is he, a woman she and a table it. With animals you have a choice. You can call a dog "it", (and I would) but a dog lover would probably prefer "he" or "she".
There's no rule saying all food is female. Pizza and beer is feminine(yay) but wine and cheese is masculine. We just have to try to memorize. It's not really about sex. Man and woman might be obvious genders, but some are counterintuitive.
For instance egg (œuf) and uterus is masculine and even though they have none, and beard (barbe) is feminine even though women don't have beards (well, most LOL) !!!
I believe the gender comes from each nouns word origin thousands of years ago.
In French the articles un/une which both mean "a" in English have to match the gender of their noun. "pizza" is a femine word so "une" is the feminine version. There's some general rules to gender in French, but a lot of exceptions, so you kind of have to learn gender as you go. Une pomme, un gâteau, une personne, un enfant. And some words can be either like a male or female artist. Un artiste/une artiste whether you're talking about a girl or guy.
"C'est" by itself is pronounced like "say," but when combined with a feminine noun that starts with "une" (like "une pizza"), it's pronounced like "setuyn pizza"). (It's awfully hard to type the sound of the French "u" when used in the feminine "une.") When "c'est" is combined with a masculine noun that starts with "un" (like "un chat"), it's pronounced like "setuh shah." The "e" at the end of the feminine "une" makes you pronounce the French "u" and the sound of "n." The masculine "un" does not have the "e," and so you sort swallow the "n" and it becomes "uh." In both cases, the "t" in "c'est" is combined with the vowel of the next word ("une" or "un") and pronounced as one word. Hope that helps.
I try to think of a context when you would say some of the things in these exercises. Here is what I got for this one: 1. In response to the look of confusion and disappointment on the face of your child when they open their biggest birthday present. You try to augment their enthusiasm by telling them what they are looking at: "It is a pizza!" 2. Aliens from space have been living with you for a while, but there are still daily opportunities to discover new wonders of Earth culture. So, in response to the alien's question, "What is this delicious smelling thing that looks like a small throw rug?", you say, "It is a pizza!" 3. Driving down the interstate and something hits your windshield with a wet "thunk". You say, "What the hell?", and your wife in the passenger side says, "It's a pizza!"