Continental flood volcanism, a massive volcanic eruption that lasted

Continental flood volcanism, a massive volcanic eruption that lasted for approximately a million years, has been suggested as the cause of the explanation of a mass extinction of marine species over two hundred million years ago.

A. has been suggested as the cause of the explanation
Continental flood volcanism can't be a cause of an explanation but rather cause for a mass extinction. It literally implies that continental flood volcanism has caused the explanation of mass extinction, that is illogical

B. have been suggested as the cause
This one has Subject-Verb Agreement problem; Continental flood Volcanism ( a singular subject)... have been (plural verb)

C. has been suggested as the cause
Correct

D. has been in the past suggested as the cause
has been in the past? "was suggested" would be okay but "has been" implies that it is still suggested whereas use of "in the past" implies that it is over...Illogical Tense

E. has been suggested for the cause
"suggest for" would be a wrong usage because it is like saying continental volcanism was suggested as an object or purpose of the cause. Illogical Meaning