Energy stored in a capacitor is given by 1/2 CV², where C is the capacitance of the capacitor and V is the voltage applied across the capacitor.

Now according to the question:

Voltage V = 100V

Energy stored in capacitor = 1J

Putting in the formula:

1 = 0.5 x C x 100²

=> 2 = C x 10

=> C = 2 x 10^-4

So the capacitance of the capacitor is 2 x 10^-4 F.