The correct answer is left. The sentence would read: “Isn’t it high time you left your office?” The phrase “high time” is typically followed by a verb in the past tense, used to indicate that something should have happened or been done by now. Here’s why the other options are not suitable:

• are leaving: This is the present continuous tense, which is used for actions happening at the moment of speaking or for planned future actions. It doesn’t fit the context of “high time”.
• do leave: This form is not grammatically correct following “high time”.
• leave: This is the base form of the verb, which is not typically used after “high time”.
• did leave: This is the past simple tense, but it’s not used in this context. The past simple would be used for actions completed in the past, not for actions that should happen now.