Step 1: Understand the usage of conjunctions.
"Although" is a subordinating conjunction used to show a contrast between two clauses. In this case, despite studying hard, the result was failure.
Step 2: Analyze each option.
- (A) Although: Correct. "Although" is used to show contrast.
- (B) So: Incorrect. "So" is used to show cause and effect, which doesn't fit here.
- (C) Because: Incorrect. "Because" shows cause, but here we are showing contrast, not a reason.
- (D) Since: Incorrect. "Since" generally refers to time or reason, not contrast.
Step 3: Conclusion.
The correct conjunction to show contrast is "Although."