Look at picture please

Keep temperature constant and increase the pressure of the reaction. The rate of reaction increases.

Explanation:

First of all, the question is asking us to design an experiment to investigate the effect of pressure on the rate of reaction hence the pressure can not be held constant since it is the variable under investigation. This eliminates the first option.

Secondly, increasing the pressure of the reaction means that particles of the gas collide more frequently leading to a greater number of effective collisions and a consequent increase in the rate of reaction according to the collision theory.

Hence the answer above.

the answer is d.) potassium