whats the optimum frequency for a PMW signal?

Arduino uno has a set number of frequencies, I am currently using a PMW frequency of 980Hz. However I am not sure if this is the optimum frequency? How do I choose the best one?

What are you using the PWM for?

for speed control of a 12V DC pump

980 should be fine. The pump will likely not start until the duty cycle reaches a decent amount, then can often be backed down some.
488 should would fine as well.
You will be using the PWM to drive a transistor or some motor drive to control the 12V switching, yes?

yes I will! thank you so much for the quick response.

I need to state why I chose 980 Hz, What should my reasoning be? Also what should the minimum duty cycle be?

If you don't want the pump to turn on, it should be 0.
If you want it on full, then 255.
If you want it at some slower speed, you'll have to determine that experimentally.
Maybe 128 will yield 1/2 the pump volume, maybe you'll need something higher to achieve that. Only you can determine that.