What is the lowest calorie birthday cake flavor?

I'm ordering my brother a birthday cake today, but he is on a diet. I wanted to get him a cake he could enjoy, without completely wrecking his diet. I was just going to get a small 8 inch one with decoration.

What is the lowest calorie birthday cake flavor: carrot, white, chocolate, or yellow?
Or would a cookie or brownie cake be lower calorie?

Thanks for the help!