the items in your comma series are "fungal infections" and "insect infestations." to keep this sentence parallel, you have to pick the answer choice that matches this organization--an adjective followed by a noun. "forest fires" is the correct answer. the other answer choices break the parallel structure; choice A adds another verb, when the verb for the comma series is "protect"; choice B throws another verb in like choice A; choice C doesn't have the same adjective-noun structure. instead it has a noun, then follows with a more in depth explanation.