In each of the following questions, choose the option that best completes the gap(s). It is high time you _____ visiting that friend of yours.

The expression "it is high time" is a fixed English idiom that signals an action is overdue and should have happened already. Grammatically, it demands the past simple tense (subjunctive mood) in the clause that follows, even though the meaning refers to the present or future.

The correct sentence is: "It is high time you stopped visiting that friend of yours."

This use of the past tense after "it is high time" is an example of the English subjunctive, where the past form expresses an unreal or hypothetical situation rather than actual past time. The speaker means "you should stop now" or "you ought to have stopped already," but the grammar requires the past form.

"Should stop," "stop," and "must stop" might seem logical based on meaning, but they do not follow the grammatical rule for this particular construction. Similar expressions that take the past subjunctive include "it is about time" and "if only" (e.g., "If only I knew").