The correct answer is C. Potometer.
A potometer is a device used to measure the rate of transpiration in plants. It consists of a glass tube filled with water, with a plant stem inserted into one end. The other end of the tube is sealed with a rubber stopper. As the plant transpires, water is drawn up the stem and into the tube. The rate of transpiration can be measured by the amount of water that is collected in the tube over a given period of time.
A hygrometer is a device used to measure the amount of water vapor in the air. It does not measure transpiration.
A cresecmeter is a device used to measure the growth of plants. It does not measure transpiration.
A clinometer is a device used to measure the angle of slope. It does not measure transpiration.