This is a rule you will have to apply VERY often on Duolingo. In French, "c'est" (sing.) and "ce sont" (plural) are used in a large variety of expressions, when a pronoun (it, she, he, they) is subject of verb "être" and followed by a nominal group, ie: determiner (+ adjective) + noun.
o it is + modified noun = c'est + determiner + noun
o she is + modified noun = c'est + determiner + noun
o he is + modified noun = c'est + determiner + noun
o they are + (modified) noun = ce sont + determiner + noun
Determiners can be: articles, demonstratives, possessives, numbers
it is a nice landscape (noun) = c'est un joli paysage
- but: it is nice (adj) = il/elle/c'est joli
he is a good friend (noun) = c'est un bon ami
- but: he is friendly (adj) = il est amical
she is a nice person (noun) = c'est une personne gentille
- but: she is nice (adj) = elle est gentille
they are green vegetables (noun) = ce sont des légumes verts.
- but: they are green (adj) = ils sont verts
There is an error in the algorithm, which allows numbers in letters to be replaced by numbers in digits to make lazy learners' lives easier. Since "un" is not only the number one, but also the indefinite article "a/an", the error pops up regularly. Please try to ignore it when you are suggested it as a correct answer.