11.
The answer is: A
Explanation
The correct option is A: A can, under Section 43 para 1.
According to Section 43 of the Indian Contract Act, 1872, when two or more persons make a joint promise, the promisee may, in the absence of express agreement to the contrary, compel any one or more of such joint promisors to perform the whole of the promise. This means that A can compel Z to pay the entire amount of Rs. 50,000/-, even if X and Y are untraceable. Z can then claim contribution from X and Y, if they are found later.
Option B is incorrect because Section 49 para 1 deals with the performance of a promise at a fixed time and place, which is not relevant to this question.
Option C is incorrect because A does not have to wait till X and Y become traceable, as explained above.
Option D is incorrect because Z is liable for the whole amount, not just one third, as explained above.