His knowledge of the subject was excellent but his classroom performance was…

Answer: extremely poor

Explanation: The conjunction 'but' introduces a contrast between the two parts of the sentence. The first part says his knowledge was excellent, so the second part must present an opposite idea about his classroom performance.

• 'extremely poor' fits because it provides a strong negative contrast to 'excellent', matching the meaning signaled by 'but'.

• 'good' is incorrect because it is positive and does not contrast with 'excellent'.

• 'desirable' is incorrect because it means appropriate or wanted, not poor performance, and therefore does not create the required contrast.

• 'praiseworthy' is incorrect because it is positive and means deserving praise, the opposite of a negative description.

Tip: Look for contrast words such as 'but', 'however', or 'yet' to determine whether you need an opposite or contrasting meaning in the blank.