The correct answer is D. Pitot tube.
A pitot tube is a device used to measure the speed of a fluid. It consists of two tubes, one facing upstream and the other facing downstream. The upstream tube is open to the fluid, while the downstream tube is connected to a pressure gauge. The difference in pressure between the two tubes is proportional to the speed of the fluid.
A hot wire anemometer is a device used to measure the speed of air. It consists of a heated wire that is placed in the air stream. The rate at which the wire cools is proportional to the speed of the air.
A rotameter is a device used to measure the flow rate of a liquid. It consists of a tapered tube with a float inside. The float rises or falls in the tube depending on the flow rate of the liquid.
A venturimeter is a device used to measure the flow rate of a liquid or gas. It consists of a constricted section of pipe called a venturi. The pressure difference between the upstream and downstream sections of the venturi is proportional to the flow rate.
Pitot tubes are the most commonly used instrument for measuring the speed of submarines. They are reliable and accurate, and they can be used in a variety of water conditions.