The phrase "so loudly that I...'' suggests that the speaker was audible due to the loudness.
Let's analyze:
- (A) "Cannot hear him'' — present tense, not suitable here.
- (B) "Was hearing him'' — awkward construction, not typically used.
- (C) "Could hear him'' — past tense, correct and natural in context.
- (D) "Should hear him'' — implies obligation, which doesn't fit.
Therefore, Option C is the best match.