The correct answer is D. Pitot tube.
A pitot tube is a device used to measure fluid speed. It consists of a tube with a small opening facing the oncoming fluid. The pressure difference between the opening and the back of the tube is proportional to the fluid speed.
A venturimeter is a device used to measure the flow rate of a fluid. It consists of a constricted section of pipe, called a throat, between two wider sections of pipe. The pressure difference between the two wider sections is proportional to the square of the fluid flow rate.
An orifice plate is a device used to measure the flow rate of a fluid. It consists of a plate with a small hole in it. The pressure difference between the upstream and downstream sides of the plate is proportional to the fluid flow rate.
A rotameter is a device used to measure the flow rate of a fluid. It consists of a tapered tube with a float inside. The float rises or falls in the tube depending on the fluid flow rate.
The pitot tube is the only instrument listed that can be used to measure the speed of an airplane. The other instruments are used to measure the flow rate of a fluid.