in spanish, gender and number always need to match! (always). For example. you already know that if a word is masculine, the word for "the" is EL (singular) and LOS (plural). Feminie is LA (singular) and LAS (plural). So "the book" would be EL LIBRO (meaning one) and "the books" LOS LIBROS (meaning more than one) "the girl" LA CHICA or the girls, LAS CHICAS, etc...
you already know this, but im explaining it this way because the same thing goes for possessive pronouns. Your, his, her, our, their, my, etc (or in the plural sense too, Yours, his, hers, ours theirs, mine). In english you can say things in a posessive plural sense two ways: example: Those are HER books, OR The books are HERS.
In spanish, just like with el, los, la las, you have to match the number of the posessive pronoun to the number of the noun. "Her book" would be SU LIBRO, whereas "her books" would be SUS LIBROS. Notice how both su and libro end in S to indicate more than one. They must always match.
same thing with every posessive pronoun:
ex: TU NUMERO / TUS NUMEROS
MI HERMANA / MIS HERMANAS
NUESTRO AMIGO / NUESTROS AMIGOS
NUESTRA AMIGA / NUESTRAS AMIGAS
you see how gender and number always match? Now for the answers to the questions:
1. su casa
a) la casa de ella
b) la casa de ustedes
why a? because there is no indication of plurality of the noun (casa). One house, belonging to one person. Why b? for the same reason. ONE HOUSE, even though it belongs to two or three of 600 people, its still only ONE HOUSE.
c is incorrect because "las casas" indicates plurality which is apparently not present in the question.
2. sus abuelos
b) los abuelos de usted
c) los abuelos de ustedes
in both b and c there is plurality. More than one. That's why it is sus instead of su because there is more than one grandparent. The fact that they are grandparents of one person or more than one person is irrelavant. Its the noun you have to consider.
a is incorrect because there is only one grandmother (abuela) in that answer instead of two grandparents (abuelos) If a was correct, the question would read: su abuela.
3. su hermano
a) el hermano de él
c) el hermano de ustedes
a is correct because it is singular. One brother of his. c is correct because again, ONE BROTHER, even though he is the brother of more than one person.
b is incorrect because the subject is plural. LOS HERMANOS, wrong!
4. su madrastra
b) la madrastra de María
c) la madrastra de ustedes
both answers only show the singular form of the noun.
a is incorrect because it says "la madrastra de nosotros" which would be NUESTRA MADRASTRA (meaning OUR, not their)
5. sus padres
a) los padres de ustedes
this is the only correct answer. PADRES of another person or people (their)
b is incorrect because the subject is singular (el padre)
c is incorrect because the posessor is wrong again. "los padres de nosotras" (or two girls) would be NUESTROS PADRES.
6. su familia
a) la familia de Pedro y Juan
b) la familia de ustedes
c) la familia de usted
all three answers are correct. They all are singular nouns belonging to someone else (su), not to me (mi) not to you (tu) not to us (nuestra), to someone else.
7. sus primos
c) los primos de Lupe
the noun is again plural. PRIMOS, and they're the cousins of someone else (sus). (see how the SUS and PRIMOS match in number?)
a is incorrect because again it indicates that the cousins belong to US (nuestros primos)
b is incorrect because "el primo de ellas" indicates only ONE cousin.
8. sus tíos
a) los tíos de Pilar
b) los tíos de ella
c) los tíos de ustedes
all three are correct for the same reason given in question 6.
if you have ANYMORE questions regarding spanish, please feel free to email me.
good luck and I hope this helped.