The correct answer is: A. attaining majority.
A minor is not liable for the debts of the firm he is a partner in. However, if he chooses to continue as a partner on attaining the age of majority, he will be liable for the debts of the firm from the date of his attaining majority.
Option B is incorrect because a minor is not liable for the debts of the firm he is a partner in, even if he is admitted to the profits of the firm.
Option C is incorrect because a minor is not liable for the debts of the firm he is a partner in, even if he chooses to become a partner at a subsequent date.
Option D is incorrect because the answer is A.