Scientists have recently discovered what could be the largest and olde

A is the correct answer. This question is about the use of present vs past participle.
present participle is used to denote a present condition that still prevails
past participle is used to denote a completed action, usually in a passive mood

The sentence is definitely talking about the filigree:

filigree [of mushrooms and rootlike tentacles] spawned by a single fertilized spore some 10,000 years ago and extending for more than 30 acres in the soil of a Michigan forest

What is b/w brackets is a prepositional phrase and remove it to make the sentence less cumbersome:

filigree spawned by a single fertilized spore some 10,000 years ago and extending for more than 30 acres in the soil of a Michigan forest

As you can see, the filigree spawned by X some 10,000 years ago. This warrants the use of past participle. Also, "spawned by" denotes passive voice which justifies the use of past participle.
As for the "extending", it is still prevailing today in the Michigan forest so you cannot use past participle "extended".

The present and past participles are used to describe the "filigree", they act as adjectives.

How do you know it is still prevailing today? You do not have any reason to believe that except your general knowledge.

Nevertheless, I still cannot see why are we eliminating b)

In your explanation you said "Choice B: extends: Incorrect. This is a verb and a verb cannot be parallel to a modifier." but the verb is parallel to another verb that is placed earlier in the sentence:

"A giant fungus that is"

So, in my opinion, b) would be a correct parallelism as it is logical that the two verbs are referring back to the subject Giant Fungus

Could someone help to explain why B is wrong?

Hi Leonaann , please re-read this answer on the first page of the thread that explains why A is correct, as well as EducationAisle 's answer immediately above that explains why B is wrong. (I am assuming that you read the topic thread.)

Although the construction is relatively rare, it is fine for a noun [the filigree] to be described with a
-- past participle [spawned by a single fertilized spore some 10,000 years ago]
and
-- present participle [extending for more than 30 acres in the soil of a Michigan forest]

Both participles are adjectives.

If you are still confused (as are many people when they answer the question -- see the stats), please state why you think that B should be correct. Many issues that involve B have been discussed, so it's hard to know what you do not understand. If I know what confuses you, it's easier for me to answer.

Please correct if I am wrong.

'Spawned' is modifying tenacles (it is acting as an adjective) and extending modifies tenacles (acting as an adjective).

The reason why extends is wrong is because this would mean that 'giant fungus is...' is parallel to 'extends' . So, here it is trying to modify the fungus instead. This parallel structure basically changes the intended meaning of the sentence right? We are supposed to modify the tenacles. That is why 'extending' is correct. Please clarify.

I have a doubt..where is the Verb for the subject "a giant fungus"...isn't verb missing for this subject

Hi,

Scientists have recently discovered what could be the largest and oldest living organism on Earth, a giant fungus that is an interwoven filigree of mushrooms and rootlike tentacles spawned by a single fertilized spore some 10,000 years ago and extending for more than 30 acres in the soil of a Michigan forest.



It is imperative to understand the meaning of the sentence in order to get the correct answer. So let’s do that first. Scientists have recently discovered something that could be the largest and oldest organism on Earth. So what have they discovered? They have a discovered a giant fungus which has qualities that make the scientists believe that this fungus could be the largest and oldest living organism. What are these qualities? This giant fungus is an interwoven filigree of mushrooms and rootlike tentacles. This fungus has been spawned by a single fertilized spore some 10,000 years ago. So this is why the scientists think that this fungus is the oldest living organism. The other quality that the fungus has is that it extends for more than 30 acres. This is the reason why the scientists think this fungus to be the largest living organism



After we have understood the meaning, let us evaluate the errors in the sentence. Now notice “spawned” is not verb here. It is verb-ed modifier that is presenting the quality of the giant fungus. In the very same way, “extending” is a verb-ing modifier that is also presenting another quality of the fungus. Hence, verb-ed modifier “spawned” and the verb-ing modifier “extending” are parallel to each other. Hence this sentence is correct as it is.

POE:

Choice A: extending: Correct.

Choice B: extends: Incorrect. This is a verb and a verb cannot be parallel to a modifier.

Choice C: extended: Incorrect. We again have a verb in the past tense that cannot be parallel to a modifier. Also the fungus is still extending. It is still there. So use of past tense is wrong anyway.

Choice D: it extended: Incorrect. Same error as in Choice D.

Choice E: is extending: Incorrect. This is a verb and a verb cannot be parallel to a modifier.

Hope this helps to understand why choice A is the correct answer.
Thanks.
Shraddha