Answer:
Each rock exerts a pushing force of 15 N.
Step-by-step explanation:
The clue here is that the astronaut pushes with a force of 15N.
To understand this better we need to recur to Newton's third law which states
For every action (force), there's an equal and opposite reaction.
So, we know that the astronaut pushed both rocks with 15N force. By Netwon's third law, both rocks exert a force of 15N back, because such reaction must be equal.
Therefore, the right answer is the last choice: Each rock exerts a pushing force of 15 N.