An apparatus commonly used to demonstrate phototropism is

The correct option is heliotropic chamber.

Let us break down why this option is correct by understanding the term phototropism and looking at the functions of all the laboratory instruments listed:

1. Understanding Phototropism:
Phototropism is the growth or movement of a plant (or plant parts) in response to a directional light source. To demonstrate or study this growth response, scientists use a specialized chamber that controls light direction. This is known as a heliotropic chamber (often referred to as a phototropic chamber), which is designed to allow light to enter from only one side, demonstrating how shoots bend toward the light source.

2. Analyzing the Other Options:
- Clinostat: A clinostat is an apparatus used to eliminate the effect of gravity (gravitropism) or light (phototropism) by slowly rotating the plant. It is primarily used to demonstrate geotropism (growth response to gravity), showing that when a plant is rotated continuously, it grows straight because gravity acts equally on all sides.
- Arc Auxanometer: This instrument is used to measure the rate of growth in length of a plant stem. It does not demonstrate tropic movements like phototropism.
- Potometer: A potometer is an apparatus used to measure the rate of water uptake by a leafy shoot, which is almost equal to the water lost through transpiration. It is unrelated to tropic growth responses.

Therefore, a heliotropic chamber is the correct apparatus specifically used to demonstrate phototropism.