We should definitely . . . . . . . . that lake. A. visit B. visits C. visiting D. visited

The correct answer is A. visit.

The verb “visit” is the most common and natural way to express the idea of going to a place to see it. The other options are not as idiomatic or natural.

• “Visits” is the third person singular present tense form of the verb “visit.” It is not appropriate to use this form in this sentence, because the subject is “we,” which is a plural pronoun.
• “Visiting” is the present participle form of the verb “visit.” It is not appropriate to use this form as the main verb of a sentence, because it is a verb form that is used to describe an action that is happening at the same time as the speaker is speaking.
• “Visited” is the past tense form of the verb “visit.” It is not appropriate to use this form in this sentence, because the sentence is about something that should happen in the future.

Therefore, the correct answer is A. visit.