Concerning the amount of writings Paul produced, a word to describe him would be

versatile

erroneous

volatile

prolific

nostalgic

The answer would be the 2nd one.

Prolific

Versatile basically means flexible, erroneous basically means wrong, volatile means having an explosive temper, prolific means producing something in large amounts and nostalgic basically means reflecting on the past. These are loose definitions, but you can clearly see that prolific makes sense because the sentence implies that Paul produced a lot of writings. That would make him prolific!