The answer is D. Thorns are an adaptation that some plants have evolved in order to discourage herbivore from eating the plant. Plants would try to survive when there are environmental pressures (herbivores eating them in this case) so they would evolve to have thorns.
It is not A because bigger leafs (more surface area for absorbing the sunlight) would help plants produce more food from photosynthesis, not thorns. It's not B because it is an adaptation, not JUST mutation, but it did happen from changing some letters in the DNA sequence to evolve. It's not C because thorns doesn't help conserve resources from getting used by others. The thorns of cactus decreases the loss of water from the sunlight because it is normally in the desert so it can survive, not protect resources.
