University students were observed to pull harder on a rope when they thought they were pulling alone than when they thought three others were pulling with them on the same rope. this best illustrates

I believe the answer is: Social loafing

Social loafing refer to a phenomenon when someone is making less effort when they are working in a group.

This feeling is developed due to the underlying assumption that his/her group would collectively taker care of his/her responsibilities, so that person could afford to work less.