Answer:

A negative sentence is the opposite of an affirmative (positive) sentence and is formed by adding a negative word like 'no', 'not', 'never' etc to the main verb.

Option A: This is a negative sentence as it contains the word, 'not'. The words, 'sometimes foolish' can be replaced by 'not always wise". Hence, the sentence, 'My friend is sometimes foolish' means the same as 'My friend is not always wise'. Option A is correct.

Option B: This is an affirmative sentence as it has no negative word. It also communicates a meaning, which is different from the original sentence. Hence, this is incorrect.

Option C: This is a negative sentence as it contains the word, 'not'. However, it communicates a meaning, which is opposite to the meaning of the original sentence. Hence, this is incorrect.

Option D is incorrect as option A is the correct answer.