Question

vigorously means :
not easy to believe
without warning
everlastingly; for all time
in an energetic manner

Explanation:

The adverb "vigorously" is related to having energy or strength. It means doing something in an energetic manner. "Not easy to believe" refers to something implausible; "without warning" means suddenly; "everlastingly; for all time" refers to something eternal.

Answer:

D. in an energetic manner