Hi Aarati,
The lessons learned register, as defined by PMI is "A project document used to record knowledge gained during a project so that it can be used in the current project and entered into the lessons learned repository." The lessons learned repository is an organizational process asset. Thus the 'best' answer would be to review the lessons learned repository, but that answer choice is not provided as an option. Therefore, in my personal opinion, I would go with OPA as the correct answer over the lessons learned register.
I am wondering if this particular question has not been updated for the 6th edition of the PMBOK® Guide. If I recall correctly, the concept of the lessons learned repository was introduced in the 6th edition. In your preparations, make sure that your study material has been updated to the 6th edition to avoid confusion.