1. My brother (b) plays football every Saturday. (My brother is a third person singular, so we use 'plays'.)
2. She (b) doesn't like apples. ('She' is a third person singular, so we use 'doesn't'.)
3. They (b) go to the beach in summer. ('They' is plural, so we use 'go'.)
4. I (a) study at home. ('I' is first person singular, so we use 'study'.)