The sentence talks about someone's confident speech and its result.
Step 1: Understand the sentence structure
The phrase "He spoke so confidently that he ...'' means the confidence led to some outcome. The correct verb form must express what he was able to do due to confidence.
Step 2: Analyze options
- (A) "Could convince anyone'': This means he was able to persuade anyone, which fits logically and grammatically.
- (B) "Was convinced anyone'': Incorrect form; passive voice does not fit here.
- (C) "Convincing anyone'': Incorrect as a filler; lacks auxiliary verbs.
- (D) "Should convince anyone'': Suggests obligation, which doesn't fit the context.
Step 3: Choose the best option
Option (A) is correct because it expresses ability caused by confidence.