Which of the following will decrease the frequency of collisions? 1. temperature
2. concentration
3. catalyst

The question isn't worded properly, but if 1 or 2 are DECREASED, the frequency of collisions of specified molecules will decrease.

Explanation:

Catalysts only facilitate reaction once molecules collide. Increased temperature makes molecules move more, and thus collide more. For concentration, if there are more molecules in the same amount of room/liquid, there will be more collisions because there are more of the molecules to collide.

a is the correct answer