Try to substitute plural pronouns in these sentences. We and they are good plural subject pronouns. If you can substitue one of these, then another subject pronoun like I, he, or she would be correct. Us and them are good plural object pronouns. If you can substitute one of these, then another object pronoun like me, him, or her would be correct.
My grandmother and I get ice cream...
Would you say “we get ice cream” or “us get ice cream”?
You would say “we get ice cream.” So a subject pronoun is correct. So this choice is correct.
As mentioned in choice A, we wouldn’t say “us get ice cream” so an object pronoun, me, wouldn’t be correct.
Use myself when the one talking is the subject and the object (ie I am getting ice cream for myself).