Step 1: Understand the conjunction usage.
"So" is the correct conjunction to use here to show cause or reason. The reason for not being able to meet is the appointment. Step 2: Analyze each option.
- (A) for: Incorrect. "For" is not the best conjunction here. It can be used, but "so" is more common for cause-and-effect situations.
- (B) hence: Incorrect. "Hence" is more formal and would not typically be used in casual conversation.
- (C) so: Correct. "So" is the appropriate conjunction to show cause or reason.
- (D) whether: Incorrect. "Whether" introduces a question or condition, which doesn't fit the context of the sentence. Step 3: Conclusion.
The correct conjunction is "so." The sentence should be "I can't meet you tomorrow, so I have an appointment."