Did you refer to the answer in OG....it's not acceptable to you?why?
For me,The explanation in OG is fine
Can you tell us which option is correct(for you)?

Hi,

Thanks for the response. I did look at the OG and the explanation is as below:

A. This sentence has the participles spawned and extending in a correct parallel contruction. Spawned refers to something that happened in the past, while extending refers to something that continues into the present.

I thought the answer could be B.

B. Extends is a present tense verb, not the participle needed for a parallel structure; the ostensible parallel between entends and the distant verb is is superficial and would result in an awkward and unclear sentence.

Couldnt quite understand what is meant by the participle required. why does it need a participle.. is it because spawned is a participle? Would that extends have been correct instead of extending?

thanks

Also. I shortened the sentence to ..

The fungus is an interwoven mushroom spawned by a spore and <the only thing i thought would fit was extends or that extends> extends for more than 30 acres.

I thought extending would be better used when if the sentence were:

The fungus was very large extending more than 30 acres.

Now when i re-red the sentence, it seems that the extending continues the idea of the large and old fungus instead of just a characteristic (that it extends over a large area)

Is this correct?

"Extending" is correct because it represents an action which is still continuing in the present + parallelism.
This sentence is in Present Perfect tense.So "extending" is required to create a parallel structure with the past participle "spawned"