Answer:
b. inheritance of acquired characteristics
Explanation:
For fifty generations wings of fruit flies were clipped. Hence they acquired this trait in their lifetime and not genetically. If acquired characteristics were capable of passing on to next generation, 50 generations would have been enough to inculcate this clipped wing trait in fruit flies. Despite it, the fifty-first generation did not have clipped wings. Hence evolution can not occur without genetic variation. A character simply acquired in lifetime does not create a difference in germ cells and hence is not enough to be passed on to next generation or cause evolution.