Explanation

The error lies in part (b). The construction ‘The reason … is because’ is redundant and ungrammatical: ‘because’ already introduces a reason, so it must not follow ‘the reason’. The standard form is ‘The reason … is that …’. The corrected sentence reads: ‘The reason for his failure is that he did not work hard.’ Parts (a) and (c) are well formed. Hence the error is in (b).