The correct option is
B became
Answer : A
Explanation:
The simple past tense is used to talk about the things that happened / existed in the past / before the present time.
Judging by the construction of the sentence ("last year") suggest that the past is being talked about.
Thus to correctly complete the sentence we need to fill the blank with the past form of the verb.
--became : past simple tense of "become"
--become : base form; past participle
--becomes : present tense (singular) of "become"
--has become : present perfect tense (singular) of "become"
Thus clearly the most suitable option from among the options is "became", as it is the past tense of the verb "become".
Therefore the correct complete sentence is:
He BECAME a Member of Parliament last year.