Explanation

Part (a), 'Beside', contains the error. 'Beside' means 'at the side of' or 'next to', whereas 'besides' means 'in addition to' or 'apart from'. The sentence means 'apart from his mother, he has two aunts', so 'besides' is required. The intended meaning is the inclusion of additional relatives, not physical position. Hence option (A) marks the error.