The gravitational force is independent of the intervening medium. In other words, the force between two masses remains the same whether they are in air, vacuum, water or separated by a brick wall.
Hence, (a) is an incorrect statement.
The gravitational force is a conservative force.
Hence, (b) is an incorrect statement.
The gravitational force obeys Newtons third law of motion. Thus, it form action-reaction pair.
Hence, (c) is a correct statement.
The gravitational force is a central force. Hence, (d) is an incorrect statement.