Statement(1):
sufficient to answer.
number of members = (total contribution)/(individual contribution) = 60/4 = 15

Statement(2):
Let the number be x, which implies each contribution must be 60/x, if all members contribute equally.
"If 5 members fail to contribute, the share of each contributing members will increase by 2" => (x-5)(60/x - 2) = 60
This yields two values for x. But without finding the values, we cannot take any decision.
After solving the equation, we get x = 6 or -10
Negtive value for x can be neglected, as it represents number of members. As there is no ambiguity in the value of x, statement(2) alone is also sufficient

Answer "D"