What is the Dominant in the C major scale?

The dominant note in any scale (major or minor) is the 5th (V) note of the scale.

I - Tonic

II - Supertonic

III - Mediant

IV - Sub-Dominant

V - Dominant

VI - Sub-mediant

VII - Leading tone or leading note

In the case of C major, the dominant note is G.
The dominant of C is G.