In a full wave rectifier circuit operating from $50\text{ Hz}$ mains frequency, the fundamental frequency in the ripple would be:

In a full wave rectifier, both halves of the AC cycle are converted into DC pulses. This means the output pulses occur twice as often as the input cycles.Ripple Frequency $=2\times \text{Input Frequency}$.
$$f_{ripple}=2\times 50\text{ Hz}=100\text{ Hz}$$