The correct option is
C has been
Answer : D
Explanation:
The construction of the sentence suggests that "he" had become an MP sometime in the past and since then up to the present time he is an MP.
To talk about something that began in the past and is continued to the present time, we use the present perfect tense of the verb.
To construct a sentence of present perfect tense, we use has(for singular) or have + the past participle form of the verb.
--was : past tense
--in being : present progressive tense
--is : third person singular of "Be"
--has been : present perfect tense
Thus, clearly the most suitable and the grammatically appropriate alternative to fill the blank is "has been".
Therefore the correct complete sentence is:
He HAS BEEN an MP since then.