23) A

Momentum = mass x velocity = 400 x 15 = 6000 kg m/s
The water ejected achieves a velocity of 15 m/s in 12 seconds.
Initial velocity = 0 m/s
Final velocity = 15 m/s
Acceleration = (final velocity – initial velocity) ÷ time = (15 – 0) ÷ 12 = 1.25 m/s2

Each nozzle ejects 400 kg of water. Two nozzles eject 800 kg.
Force = mass x acceleration
Force = 800 x 1.25 = 1000 N