The third one is the right answer.
Mutations appear randomly and these mutations can either have possitive, negative or neutral impact on the individual's survival. The environment of the individual amongst other factor determine the net effect of the mutation. For example:
A mutation on a shade plant for bigger leaves would have a possitive impact on the plants survival due to the increase of the photosynthetic tissue area. On the other hand the same mutation on a desert plant would have a negative impact on the plant's survival due to the increase of the leaf area which leads to increased water evaporation off the plant.
Answer:
yes it is a useful mechanism
