Answer:
I'm pretty sure it's the 4th.
Explanation:
Even though you didn't give us the fragment with the comma slice, it isn't really necessary here as we can answer just based on these 4 options here. All options apart from the second one are correct - only the second option doesn't fix the mistake of the comma splice, whereas the other options do but connecting the sentences properly.
Answer:
C) as if there had been some magic power in the vigorousness
as if there had been some magic power in the vigorousnessof his speech