The correct option is A believe
Here, 'believe' in Simple Present Tense is correct because
it is used to indicate a future event on a designated date as part of a
plan or to express habits or general truth. Here, it is a general truth believed by many that Mohammed Ali is the greatest boxer of all times.
'Are believing' in present continuous is incorrect because it implies
an action going on at the time of speaking.
'Believes' is incorrect because of subject-verb disagreement. 'Believes' is singular, but in disagreement with plural 'many people'.
'Do believed' is in past tense, but the sentence is in present tense, as indicated by 'is' in the sentence.