Which change will decrease the number of effective collisions during a chemical reaction?

The correct option is this: DECREASING THE TEMPERATURE.

Explanation:

During the process of chemical reaction, the reacting molecules typically move about randomly in the reacting vessels colliding with one another and with the wall of the container. The rate of movement of the reacting particles can be reduced by decreasing the temperature of the reaction. If the temperature is reduced, the particles will lose kinetic energy and thus, they will move more slowly and collide less often with one another.

i believe it is-

d.) gravity pulled particles of dust and gas together to form planets.