Talking about stress

C, the root cause of liquid pressure is gravity.

What you said in A is complete, because although liquid is flowing, if there is no gravity, there will be no pressure. For example, if a liquid is taken into space, it will "shrink into a ball" but there will be no pressure.

In fact, strictly speaking, it should be subjected to external forces. If you have studied hydraulics and transmission, you will understand that in hydraulic transmission, the thrust of liquid (provided by hydraulic pump) is much greater than gravity, but at this time, the nature of pressure in liquid remains unchanged.