"The woman writes to the boys."
Translation:La donna scrive ai ragazzi.
To be more specific: "al"=a+il, while "ai"=a+i
You use "ai" on "ragazzi" because it takes the "i" definite article.
"zii" (uncles), for example, is also plural, but you use "agli"(=a+gli) on it as a prepositional article.
Similarly, you say "allo zucchero", "all'uovo" (both singular), etc.
The same goes for all other prepositional articles:
• nel=in+il, nella=in+la, nello=in+lo, negli=in+gli..
• del=di+il, delle=di+le, degli=di+gli..
• sul=su+il, sull'=su+l', sui=su+i..
I got tripped up on this, too, but figured out the problem. The example sentence was 'They are chocolate cookies.' Why was the correct answer using 'al' vs 'ai'? Then it occurred to me, chocolate is not plural, but cookies is. So the correct answer is 'Sono i biscotti al cioccolato'.
In Italian when you have a group of all girls, you use ragazze. If it's all boys or a mix of boys and girls you use ragazzi.
This goes for all group words in Italian. A group of 5 women and 1 man is grammatically masculine. Without context you wouldn't immediately know whether there are girls in the group or not.