The correct option is A Auxanometer
Auxanometer is used to measure the growth in length of a plant organ, there are two types of auxanometer. i.e. Arc auxanometer and Pfeffer's auxanometer (automatic auxanometer). Auxanometers can measure plant growth in response to a variety of parameters, including temperature, sunlight and water conditions. It can also be used in the laboratory or in field, to understand the effect of different factors on plant growth. Some sensitive auxanometers can measure growth as little as a micrometer.