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