It's

Power = Capacitance * (Voltage ^2) * frequency

The difference in energy stored in a capacitor at two different voltages is 0.5*C*(V^2). A chip will burn that energy 2*(frequency) times per second, so P=C*(V^2)*F.

Edit: forgot a half

The latter. See also:

https://en.wikipedia.org/wiki/Processor_power_dissipation#So...

and

https://electronics.stackexchange.com/a/33340

In general you can often work out the answers to questions like this by considering units. In this case, we don't need to figure out what Watts / Farads is(1), we can see that Volts^Hz is not going to give us anything well-behaved.

(1)[m^4 kg^2 s^-7 A^-2]