The weight of the load is (mass) x (gravity)
= (350 kg) x (9.8 m/s²)
= 3,430 newtons .
The force on the surface of the liquid is the weight of the piston and its load.
The pressure on the surface of the liquid is (force) / (area)
= (3,430 newtons) / (1 m²)
= 3,430 pascals
= 3.43 kPa
This doesn't include anything for the weight of the piston without its load,
but it's certainly the closest of the choices.
