Because adjectives in Italian usually agree with the noun they describe.
Here, bianca describes camicetta, which is:
So the adjective must also be feminine singular:
So:
Yes, but it means something slightly different in use.
Italian often prefers di + name when it wants to be especially clear about whose object it is.
Nell’ is a contraction of:
So:
This happens because armadio is masculine singular and begins with a vowel, so its article is l’:
After the preposition in, the combined form becomes nell’.
Armadio is a very common word meaning wardrobe, closet, or cabinet, depending on context.
In this sentence, nell’armadio most naturally means:
Guardaroba exists too, but it is used differently and can mean things like:
So armadio is the normal everyday word here.
A simple pronunciation guide is:
La ca-mi-CHET-ta BIAN-ca di SA-ra è nel-lar-MA-dio.
A few helpful notes:
If you want to sound natural, don’t pause too much: La camicetta bianca di Sara è nell’armadio.