Which one of the following is the correct comparative from of the given sentence? “You are as big a fool as a donkey”?

The original sentence expresses equality: you and the donkey are equally foolish, as shown by the 'as ... as' construction. The standard transformation converts this positive equality into a negative comparative with the subject and object swapped. So 'A donkey is not a bigger fool than you' means the donkey is not more foolish than you, which leaves you at least as foolish as the donkey. That matches the original, so C is correct.

• A says the donkey is less foolish than you, which makes you more foolish. That turns the equality into a claim of superiority.
• B has the same flaw. 'Not as big a fool as you' places the donkey below you, again asserting that you are the bigger fool.
• D reverses the meaning completely by making the donkey the bigger fool.

The core concept is the conversion of degrees of comparison: positive equality with 'as ... as' can be restated as a negative comparative with 'not ... than'. This is a staple of the English language component of PCS papers, and the same logic applies to sentences such as 'He is as wise as he is old' type transformations.

Trap: Candidates choose B because it looks like a direct negation of 'as ... as'. They miss that the original asserts equality, not superiority, and that the subject and object must be swapped in the restated form.