What is the correct answer?
At the instantaneous center of rotation, one rigid link rotates instantaneously relative to another for the configuration of mechanism considered
The two rigid links have no linear velocities relative to each other at the instantaneous centre
The two rigid links which have no linear velocity relative to each other at this center have the same linear velocity to the third rigid link
The double centre can be denoted either by O2 or O12, but proper selection should be made