Translation:As calças dela
Why isn't "as suas calças" correct, if "suas calças" is an acceptable answer?
Both the singular and plural are commonly used. I hear one more than the other depending on the situation and the words used.
If you're being specific about the pants you're wearing you'll probably say "vou vestir a calça", but if you're just trying to say you're gonna put some pants on and go out you'll probably say "vou vestir as calças".
With the indefinite article, I hear people say "uma" more than "umas", probably because the word "uma" also means one, and "umas" mean some or a few, so it can sound strange to say "vou vestir umas calças" (I'm going to wear some pants), unless you're really trying several pants.
It's a complex rule. Seu/sua is the possessive pronoun of the 3rd person, which is usually he/she. But in Portuguese (specially Brazilian Portuguese), since the colloquial pronoun "você", which is conjugated in the 3rd person, is widely used, there are a lot of ambiguous cases. To avoid ambiguity, the form dele/dela (meaning his/her) is preferred instead of seu/sua (and they're always placed after the noun), while the word seu/sua is more commonly used to mean your.
That is, unless the sentence has a known subject, so the possessive can agree with it, like in the phrase "Ela esqueceu suas calças." (She forgot her pants). In this case, since we've identified that the subject of the sentence was "she", we know the pronoun "suas" means "her" and not "your" (it could still mean your depending on the context though). But this is more commonly seen in texts, in speech people usually say dele/dela or omit it when the context is clear.
Phew. I hope that didn't scare you off.