What is required to overcome the resistance to flow in pipes and fittings?

Pressure is the correct choice because it is the driving force that moves water through pipes and fittings in a hydraulic system. In fluid dynamics, pressure is needed to overcome the inherent resistance that occurs due to friction between the fluid and the walls of the pipe, as well as between the fluid and any fittings, valves, or other obstacles within the system. This frictional resistance can cause energy loss, which must be compensated for by supplying sufficient pressure to maintain the desired flow rate.

Adequate pressure ensures that the water can flow effectively and reach its destination, whether it's a faucet, a sprinkler system, or any point of use in the water distribution network. If the pressure is too low, it may result in insufficient flow or even a complete stoppage of water movement, affecting the efficiency and reliability of the system.

The other terms, while important in different contexts, do not specifically address the resistance to flow in the same way. Voltage is associated with electrical systems, torque pertains to rotational forces in mechanical systems, and horsepower is a measure of the rate of work done or energy transfer. None of these directly relates to fluid flow and overcoming hydraulic resistance in pipes and fittings.