The answer is: A
Explanation
The correct option is A: Mr. A may treat his joint-decree as cross-decree under Order 21 Rule 18.
The explanation is as follows:
Order 21 Rule 18 of the Code of Civil Procedure deals with the execution of cross-decrees, which are decrees passed between the same parties and capable of execution at the same time by the same court. According to this rule, if the two sums due under the cross-decrees are equal, satisfaction shall be entered upon both decrees. If the two sums are unequal, execution may be taken out only by the holder of the decree for the larger sum and for so much only as remains after deducting the smaller sum, and satisfaction for the smaller sum shall be entered on the decree for the larger sum as well as satisfaction on the decree for the smaller sum.
In the given question, Mr. X, Mr. Y and Mr. Z are jointly and severally liable for Rs. 10,000 under a decree obtained by Mr. A. This means that Mr. A can recover the whole amount from any one of them or from all of them in proportion. Mr. Y obtains a decree for Rs. 10,000 against Mr. A singly and applies for execution to the Court in which the joint-decree is being executed. This means that Mr. Y can recover the whole amount from Mr. A alone. Therefore, Mr. A and Mr. Y have cross-decrees for the same sum of Rs. 10,000. According to Order 21 Rule 18, satisfaction shall be entered upon both decrees and neither of them can execute their decree against the other. Mr. A may treat his joint-decree as cross-decree under this rule and inform the court of such payment or adjustment.