Explanation:

Subject-Verb Agreement:
In this sentence, the subject is "we" and the verb is "to visit". The subject "we" is the first person plural pronoun, which indicates that the verb should be in the present tense. Therefore, the correct verb form is "go".

Simple Present Tense:
The simple present tense is used to describe a habitual action or a general truth. In this sentence, the action of visiting the grandparents is described as a regular occurrence every year. Therefore, the verb "go" should be in the simple present tense.

Options Analysis:
a) go - This is the correct answer as it is in the simple present tense and agrees with the subject "we".
b) goes - This option is incorrect because it is in the simple present tense but does not agree with the subject "we". "Goes" is used with the third person singular pronouns like "he", "she", or "it".
c) went - This option is incorrect because it is in the past tense. The sentence is talking about a present or future action, so the past tense is not appropriate.
d) are going - This option is incorrect because it is in the present continuous tense. The sentence is not describing an action happening at the moment, but rather a habitual action that occurs every year.

Therefore, the correct answer is option 'A' - go.