Regarding NOC for competitive exam

Is that comment written on th NOC letter given to you? I don't think so. He might have written that comment on the letter written by you asking for NOC as a comment while forwarding it for necessary action. There is no such rule in government that when we are getting relieved from a government post we have to show them a replacement. That note is not for you. You need not worry. You can get ready for the competitive examinations and if you get a better post you can resign to this within the terms and conditions of your employment. Don't keep any unnecessary thoughts and concentrate on preparing for examinations.

Thank a lots

NOC is kind of certificate indicating that the department has no objection to relieve you from the present assignment if selected. This is issued by the personnel department after getting the recommendation of your mother department.
You might have seen such a remark in your application requesting for a NOC.
The comment appearing on that application is not likely to affect your process of relieving if you successfully pass the competitive examination and ultimately you are selected.
The remark is just the internal arrangement to be made by the organisation to select an appropriate candidate in the event of your relieving.
In the event of your selection, tender your resignation and join the new post.

The comment quoted by you in the question "replacement is necessary in case of selection" is relevant between the NOC sanctioning authority/department and your department .It is not a part of the NOC letter, but a comment on the recommendation and/or forwarding letter to the NOC sanctioning authority as per your department/organisation protocol.
You need not mistake it as a condition or responsibility on you to provide a replacement or substitute.
Why such comments are mad in such recommendations or forwarding letters?
There are two reasons. 1. In case such an employee is selected for the applied job it will be the duty and responsibility of the NOC giving office to ensure the employee is relieved on time to join the new assignment. Then that will create a shortage of stave in the concerned office. Their work will suffer. Unless they make such a comment and notify the higher authority that they need a substitute/replacement staff they will not get it. It will be assumed that they have excess staff. No department will like to create such assumption.
2. In case they do not write such a command, that will create an impression that the NOC applying employee is not worth to be retained and they are happy to get rid of him. That is not nice to the employee's reputation. So every department will put such a comment even if they can relieve him by re-allocating his work among existing staff and manage.
Take it as only a usual procedure. You will get a NOC letter in the standard format after due procedure.
If not, you have the right to resign subject to the terms and conditions in your appointment and the organization's rules and regulations.