Answer: option is C: the cross-section of the water decreases.
Step-by-step explanation:
From the equation of continuity for in-compressible fluids (Bernoulli's equation),
a v = constant
where, a is the area and v is the velocity.
Also, pressure is inversely proportional to the velocity.

From the image, area of cross-section at A is greater than at B,

thus,

which means the velocity of water at B would increase and pressure of the water decreases. Hence, the correct option is C, i.e. the cross-section of the water decreases.