Correct choice is (c) The sum of two inputs must be equal to responses to these signals
Easiest explanation: The sum of two inputs must be equal to responses to these signals individually.
x[n]*(h1[n]+h2[n])=x[n]*h1[n]+x[n]*h2[n], with x[n] being the input and h1[n] and h2[2] being the impulse responses.