Step 1: Understanding the sentence.
The sentence presents a contrast. It mentions that the test was challenging, but despite this, the speaker managed to finish on time. This contrast is best expressed by using the word "but."
Step 2: Why other options are incorrect.
- (A) "since" implies causality, which doesn't fit here as there is no cause-and-effect relationship.
- (B) "because" also suggests causality, which doesn't match the intended contrast.
- (D) "although" would fit the context but grammatically doesn't work as smoothly with the structure of the sentence.
Step 3: Conclusion.
The correct connective is "but," which contrasts the difficulty of the test with the successful completion of it.
Final Answer: but.