A comparison is made in the given sentence between Ajeet andhis brother in terms of being a scholar. ‘Bigger’ is not suitablebecause it is used in the sense of size. When abstract things arecompared then we use adjectives of quality. So, either ‘better’ or‘superior’ is to be used. But, ‘superior’ can’t be used becausethe use of ‘than’ is there in the sentence. Hence, ‘bigger’ needs tobe replaced with ‘better’. Thus, option (a) is correct.