Hi Chaitanya,

That's a good question, because you're right that "spawned" is a past participle whereas "extending" is a present participle. However, the important thing is that participial phrases (and for that matter, prepositional phrases, too) frequently function as adjectives or adverbs. Here's a shorter example:

The woman with a hat on her head ran --> The hatted woman ran... "hatted" is a past participle functioning as an adjective describing the woman; we know it's an adjective because it can easily be replaced with a different adjective, like "pretty."

The woman ran while shouting a warning --> The woman ran shouting ... "shouting" is a present participle functioning as an adverb modifying "ran"; we know it's an adverb because it can easily be replaced with a different adverb, like "carefully."

In your sentence,

mushrooms and rootlike tentacles spawned by a single fertilized spore some 10,000 years ago and extending for more than30 acres

both "spawned..." and "extending..." are functioning as adjectives describing the mushrooms and tentacles, so we can say they're parallel. I think on this question, the only other answer that might be tempting is (C) -- "extended" -- because it looks more parallel to "spawned," but in fact, we must rule it out because of its inability to function as an adjective describing the mushrooms and tentacles.

The thing with parallelism is that it's only medium-picky. It's more like someone who says two things are alike because they're both fruits than someone who says these things are different because one is an orange and one is a banana. As long as you've got two things functioning as adjectives, you'll satisfy the demands of parallelism, even if their more specific functions are slightly different.

Hope that helps!