Complete the sentence: If he ______ more careful he would not have made that mistake.

The correct answer is had been. This sentence uses the third conditional structure, which is used to talk about hypothetical situations in the past and their hypothetical results. The structure is 'If + past perfect (had + past participle), would have + past participle'. In this case, 'had been' correctly forms the past perfect tense for the 'if' clause.

• will be is incorrect as it refers to the future and does not fit the past conditional context.
• is is incorrect as it refers to the present tense and does not fit the past conditional context.
• was is incorrect because while it is past tense, it is simple past, not past perfect, and therefore does not fit the third conditional structure required by 'would not have made'.