The phrase "c powder" typically refers to a type of powdered form of vitamin C. It is a commonly used term for describing a dietary supplement that contains vitamin C in powdered form. Full definition
They actually sold Vitamin C powder in large quantities, but the price was quite high, about 125 $ for 1 kg. (perfecthealthdiet.com)
I used only 2/3 c powdered sugar in the icing, and nobody minded at all. (skinnytaste.com)
Another easy alternative is just mixing Vitamin C powder with water and a little juice, though this can be a little acidic during high intensity exercise! (wellnessmama.com)