The structure "so [adjective/adverb] that...'' is used to show cause and effect.
Here, "studied so diligently that...'' implies a definite result.
Let's evaluate the options:
- (A) "May have passed'' — suggests uncertainty, but the sentence implies a certain result.
- (B) "Passed the exam'' — this fits perfectly. It’s in the past tense and shows a direct result.
- (C) "Passing the exam'' — grammatically incorrect in this context.
- (D) "Had passed'' — past perfect is not needed here because there’s no second event for comparison.
Hence, Option B is correct.