Explanation:

- Exceptional means unusual or extraordinary, standing out due to unique qualities.

- Option 1: Uncommon means something not commonly found, which is not quite the opposite of exceptional but rather a slightly different quality.

- Option 2: Unthinkable refers to something inconceivable, not aligning directly as an opposite for exceptional.

- Option 3: Unimaginable involves something hard to think or conceive, not directly opposing exceptional.

- Option 4: Unremarkable means ordinary or not noteworthy, closely opposing exceptional as it signifies blending in rather than standing out.

- Therefore, the antonym of exceptional here is unremarkable.