Root pressure is measured by
a) Potometer
b) Barometer
c) Manometer
d) Auxanometer

Hint: This instrument is a mercury-filled glass column. It is placed on a cut plant stump to measure the root pressure.

Complete answer:
Root pressure is the pressure caused due to the active absorption of mineral salts leading to a passive flow of water into the roots from the soil solution. The instrument used for measuring root pressure is a manometer.

Additional information
In plants, the force that helps to drive fluids upward into the water-conducting vessels (xylem) is called root pressure. This pressure is developed by osmotic pressure in the cells of the roots. Exudation of fluid when the stem is cut off just above ground demonstrates the phenomenon. Also, root pressure is higher than atmospheric pressure.
A potometer is a scientific instrument that is used for measuring the rate of water uptake of a leafy shoot which is approximately equal to the water lost through transpiration. Thus, it measures the rate of transpiration.
A barometer is a scientific instrument that is used to measure atmospheric pressure, also called barometric pressure.
An auxanometer is a scientific instrument that is used to measure the increase or rate of growth of plants.

So, the correct answer is ‘Manometer’.

Note:The theory of Root pressure was put forward by Priestley in 1916.
- The manometer was discovered by Otto von Guericke in 1661 after the discovery of the vacuum pump.
- Potometer is also known as a transpirometer.