He impresses his boss with his performance.

The other options are incorrect because they do not make sense in the context of the sentence.

• Option B: His performance impresses his boss. This is incorrect because the subject of the sentence is “he,” not “his performance.”
• Option C: His boss is impressed at his performance. This is incorrect because the preposition “at” is used to indicate a location or position. In this sentence, the boss is not impressed at a specific location or position, but rather by the employee’s performance.
• Option D: His boss is impressed in his performance. This is incorrect because the preposition “in” is used to indicate a location or position. In this sentence, the boss is not impressed in a specific location or position, but rather by the employee’s performance.

In conclusion, the correct answer is: A. His boss is impressed by his performance.