The correct option is
A was
Answer : A
Explanation:
The time frame, i.e. 1989-95, tells us that the "He" was the Defense Minister during that time in the past.
And to talk about something that happened in the past / before the present time, the past tense of the verb is used.
--was : past tense of "be"
--is : third person singular present of "be"
--has been : present perfect form of "be"
--were : second person singular of "be"
Thus, clearly the most suitable and grammatically correct alternative is "was".
Therefore the correct complete sentence is:
He WAS the Defense Minister from 1989-95.