Why is potassium hydroxide used in the bell-jar experiment involving photosynthesis?

The correct option is: To absorb carbon dioxide.

Step-by-Step Explanation:

1. Understanding the Bell-Jar Experiment:
The bell-jar experiment (Moll's half-leaf experiment setup) is designed to demonstrate that carbon dioxide (${\text{CO}}_2$) is an essential raw material required by green plants to carry out photosynthesis.

2. Role of Potassium Hydroxide ($\text{KOH}$):
Potassium hydroxide ($\text{KOH}$) is a strong chemical agent that chemically reacts with and absorbs carbon dioxide gas present in the surrounding air inside the enclosed bell jar or flask.

3. Experimental Control and Observations:
• By placing a solution or pellets of potassium hydroxide inside the sealed bell jar, all available carbon dioxide gas inside the enclosed space is completely absorbed.
• The plant or leaf inside this bell jar is provided with light, water, and chlorophyll, but lacks carbon dioxide.
• When tested for starch using an iodine solution after several hours, the leaves kept in the presence of $\text{KOH}$ show a negative starch test (no blue-black coloration), proving that photosynthesis cannot occur in the absence of carbon dioxide.

Therefore, potassium hydroxide is used in this experiment specifically to absorb carbon dioxide and create a carbon dioxide-free environment.