The correct answer is option 'A' - daren't.

Explanation:
- The word "daren't" is a contraction of "dare not."
- In this context, it means that the person is not brave enough or does not have the courage to tell his father the truth.
- The word "daren't" is used when someone is hesitant or afraid to do something.
- In this case, the person is afraid or hesitant to tell his father the truth, which suggests that there may be consequences or negative outcomes associated with revealing the truth.
- The use of "daren't" implies that the person feels a sense of fear or apprehension about the situation.
- By choosing option 'A', it suggests that the person lacks the courage or confidence to be honest with his father.

Other options explained:
- Option 'B' - can't: This option implies that the person is unable to tell his father the truth, but it does not convey the sense of fear or hesitation that is present in the original sentence.
- Option 'C' - may: This option suggests that the person has the choice or possibility to tell his father the truth, but it does not convey any sense of fear or hesitation.
- Option 'D' - must: This option implies that the person is obligated or required to tell his father the truth, but it does not convey any sense of fear or hesitation.