1. The correct answer is C. ella.
The pronoun <em>ella </em>means <em>she </em>in Spanish. Given that you are talking about one of you grandmas, the pronoun has to be singular (so, <em>ella </em>instead of <em>ellas</em>), and given that your grandmother is a woman, you need a female pronoun, <em>ella. Yo </em>means <em>I, </em>and <em>vosotras </em>means <em>you </em>(but plural).
2. The correct answer is C. él.
The pronoun <em>él </em>means <em>he </em>in Spanish. Given that you are talking about one person only, the pronoun has to be singular (so, <em>él </em>instead of <em>ellos</em>). Also, given that Marcos is a man, you need to use the male pronoun <em>él. Ustedes </em>means <em>they </em>(plural, and polite form).
4. The correct answer is D. ustedes.
The pronoun <em>ustedes </em>means <em>they </em>in Spanish, however, it is not a regular <em>you - </em>it is the polite version of the plural noun <em>ellos/ellas. </em>Since we are talking about someone's parents here, you want to address them a bit more formally than your friends, and that's why you say <em>ustedes. Ellos </em>could be the correct answer if both of your parents are men; if not, <em>ustedes </em>should be used.
5. The correct answer is A. ellas.
The pronoun <em>ellas </em>means <em>they </em>in Spanish. Given that you are talking about more than one person, you need to use the plural pronoun, which is <em>ellas </em>(insted of <em>ella</em>). Also, given that both individuals are girls, you need to use the female pronoun <em>ella. Vosotras </em>means <em>you </em>(plural), and <em>nosotras </em>means <em>we </em>(plural).