The correct option is B {1, 2, 3, 6}
Sample space for the dice thrown once = {1,2,3,4,5,6}
B = {4,5}
So B’ = S - B
= {1,2,3,4,5,6} - {4,5}
= {1,2,3,6}
Please note that complement of an event is not defined with respect to some other event. It is only with respect to sample space. So, elements of the event B are to be subtracted from sample space and not from A.