From (a), (c) and (d), and as there are only five boys.
C and D are sitting together, and B is two places away from E. This is possible only when one of B and E is sitting in the middle of the row and the other sitting at any of the extreme ends of the row and A is sitting between B and E.
From (b) and (f), we can say that B cannot sit at any of the extreme ends of the row.
B is sitting at the middle of the row and E is sitting at one of the extreme ends of the row.
We have two possibilities:
Case (1) CDBAE
Case (2) EABCD
Consider case (1):
From (e), C entered second, from (f) A and D would have entered third and fourth respectively.
E entered first, which violates condition (f).
Case (a) is not possible
Consider case (2):
From (e) E entered second.
From (f), A and D would have entered third and fourth respectively. Hence, C entered first.

C entered first.