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 root-like 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.

Th e original sentence is correctly written. Th e giant fungus is described as an interwoven filigree spawned … some 10,000 years ago and extending for more than 30 acres. Th e present participle extending parallels the past participle spawned. A Correct. Th is sentence has the participles spawned and extending in a correct parallel construction. Spawned refers to something that happened in the past, while extending refers to something that continues into the present. 
option A is the answer

 "EXTENDING"