Answer:
a.B and C
b.B is a subset of A
Explanation:
We are given that a coin is tossed four times
Sample space={HHHH,HTTT,HHTT,HTHT,THTH,TTTH,TTHH,HTTH,THHT,HHHT,THHH,TTTT,HHTH,TTHT,THTT,HTHH}
A:Exactly two heads appear
A:{HHTT,HTHT,THTH,TTHH,HTTH,THHT}
B:Heads and tails alternate
B={HTHT,THTH}
C:First two tosses are heads
C={HHTT,HHTH,HHHH,HHHT}
a.We have to find mutually exclusive events
Mutually exclusive events: It is that events in which intersection of events is empty.
Event B and C are disjoint because

Hence, B and C are mutually exclusive.
b.We have to find that sets which are subset of other.
We can see that B is subset of A because all elements of B contain in set A.
Therefore , event B is a subset of event A.