Maybe this will help:
Neither spawned nor extending is the main verb of the sentence. You should not look for both of these to be in the same tense.
It is best to think of these verbs as functioning to introduce modifiers, and those two modifiers should be parallel to each other.
If we think of this as I have suggested above, the two modifiers are
spawned by a single fertilized spore some 10,000 years ago
and
extending for more than 30 acres in the soil
.
How can you tell if two grammatical structures are parallel? Very frequently you can do this by ensuring that he parallel words share the same ending. However, there are numerous examples where that isn't enough.
I suggest you try to read the entire sentence with each grammatical structure alone in the sentence. So for this question we could consider the following.
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 in the soil of a Michigan forest.
AND
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
extending for more than 30 acres in the soil of a Michigan forest.
Now try to read the second sentence above with any of the other answer choices:
B.
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
extends for more than 30 acres in the soil of a Michigan forest.
Clearly the verb
extends doesn't function as a modifier. This is easy to point out from both a grammatical point of view and a "what sounds right" point of view.
C.
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
extended for more than 30 acres in the soil of a Michigan forest.
Now the verb
extended is also nonsensical.
D.
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
it extended for more than 30 acres in the soil of a Michigan forest.
The pronoun
it is obviously unnecessary and unclear.
E.
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
is extending for more than 30 acres in the soil of a Michigan forest.
The verb
is extending doesn't make much sense since we don't know what is extending.
I think it is useful to remember that parallelism is a form of compounding sentence constituents. You should always be able to break each of those constituents out of the sentence and consider the complete sentence with each constituent separately.
This is a useful way to get the answer whether you prefer an approach based on grammatical rules or an approach based on what sounds right.
I hope that is helpful.
