The correct answer is D. is leaving.
The sentence is in the present progressive tense, which is used to describe actions that are happening right now. The other options are not in the present progressive tense.
A. leaves is in the simple present tense, which is used to describe actions that happen regularly or repeatedly.
B. leaving is the present participle of the verb “to leave,” which is used to form the present progressive tense.
C. does leave is in the simple present tense with the auxiliary verb “do,” which is used to ask questions or make negative statements.
In this case, the bus is leaving right now, so the present progressive tense is the most appropriate tense to use.