The correct option is B Calculating rate of growth
Auxanometer is an instrument used to measure the increase in growth in plants.
The apparatus in arc auxanometer is set up in such a way that an initial value of the parameter is demarcated, which is known as the reference point.
This is done using a pointer which is focussed onto the initial point and left.
The displacement of the pointer from its original position to a new position (due to growth of the plant such as increase in its length) across the arc scale is measured and calculations are done to measure absolute and relative growth.