The correct answer is: was.
The verb “was” is the past tense of the verb “to be”. It is used to describe a state or condition that existed in the past. In this case, the state or condition is that the person was the Defense Minister.
The other options are not correct because they do not describe a past state or condition. The verb “is” is used to describe a state or condition that exists in the present. The verb “has been” is used to describe a state or condition that began in the past and continues into the present. The verb “were” is the plural form of the verb “to be”. It is used to describe a state or condition that existed in the past and involved more than one person.
In conclusion, the correct answer is “was” because it is the past tense of the verb “to be” and it describes a state or condition that existed in the past.