Answer: Diffusion of responsibility
Step-by-step explanation:
Diffusion of responsibility is the phenomena in which people tend not to make a decision or any action in front of large group of people .They don't react on particular situation as they think some other person will take the correct step instead.
According to situation mentioned, Nabila's decision was based on was made on diffusion of responsibility .She did not took any step for man on ground as she thought crowd members must have called for help already .She was facing pressure in acting by herself for helping the man.