Hello Tushar,
This is a minor issue and it will not cause you any problems on the PMP exam.
But the answer to your question is very unsatisfactory: Nobody agrees on what the P in PMO stands for. About 50% of project managers says it stands for "Project" and the other 50% say that it stands for "Program".
(I also heard project managers say that PMO stands for "Piss Me Off" or "Provides More Opportunity"...
In the end, the question isn't what the P stands for but what the PMO actually does. So as long as you understand what the function of a PMO is according to the PMBOK Guide then you will do just fine.