How would you deal with your subordinate, who got demotivated for some reason and wants to resign from the job?

Take him/her for a lunch or dinner.

Understand the reason in detail, workout a solution for that , motivate him/her with examples.

Show the career path and opportunities within the organisation.

Create sense of care and support.

Give him/her few days break.  

Hope it will work to motivate the employee again .

second option:-  Understand the reason , if it is genuine and you feel that  he/she can not performance within the given circumstances. Let him/her resign and advice a better career approach.   

Meeting with him. Informal one will be great to get in depth of actual problem he/ she is facing. Understanding his/ her issues. Evaluating what I can do or my organization can do regarding that. Then will find out some good solution to motivate him/her. no doubt will try to retain him/her.

Thanks 

I have to know the reason and meet him alone then accept or prevent him 

I agree with our specialists here...

I will deal with him directly and objectivity . first discuss the basic issues that made him demotivated and try with each other to Proposing solutions and alternatives 

Thanks for invitation,

I will invite him for a informal meeting to discuss his reasons and point of view and will friendly advise him without any pressure from my side, and leave the official "Exit- meeting" to the HR Department.  

Sorry i cannot deal without knowing the reason 

Managers and leaders need to understand their empoyees and having social and emotional intelligence is key here. If you know what drives your employees, then you would sense timeously for wrong signlas. In this case we need to pull the right levers and have a one on one to to determine what the cause is. Tap into his emotions as this will help you to make a rational decision. Give encouragement, support and retain for the right reasons.