Explanation:

This is an indirect question. The correct structure for an indirect question is 'question word + subject + auxiliary verb + main verb'. The phrase 'I do not know' introduces an uncertainty, making 'can be' (expressing possibility) more appropriate than 'will be' (expressing future certainty). Option (B) 'What the reason can' when combined with the trailing 'be' forms 'What the reason can be', which is grammatically correct and fits the context.