Concept:The phrase "it is high time" is followed by a past tense verb, but it refers to a present or future action.
This structure is used to say that something should be done now or very soon.
Explanation:The pattern is:
It is high time + subject +
past simple.
Here, the subject is "you", so the verb must be in the past simple form.
The past simple form of "leave" is
left.
The sentence means: "You should have left already, and it is time to leave now."
Option B,
leave, is not possible because the structure requires a past verb.
Option C,
hadleft, is in the past perfect and is not used after "it is high time".
Option D,
haveleft, is in the present perfect and does not fit the pattern.
Therefore, the only correct word is
left.
Answer:A. left