Introduction:
In a center-tapped full-wave rectifier, the input sine wave is converted into a pulsating DC waveform. The output ripple frequency refers to the frequency of the fluctuations or variations in this pulsating DC waveform.

Explanation:
To understand why the output ripple frequency is 100Hz, let's break down the process step by step:

1. Input Waveform:
The given input waveform is a sine wave with a frequency of 100Hz and an amplitude of 250. It can be represented as 250sin(100t).

2. Center-Tapped Full-Wave Rectifier:
In a center-tapped full-wave rectifier, a center tap is taken from the secondary winding of the transformer. The two halves of the secondary winding are connected to the diodes in a bridge configuration. This arrangement allows current to flow in one direction during positive half-cycles and in the opposite direction during negative half-cycles.

3. Rectification Process:
During the positive half-cycle of the input waveform, the diode connected to the top half of the secondary winding becomes forward-biased, allowing current to flow through it. Simultaneously, the diode connected to the bottom half of the secondary winding becomes reverse-biased and blocks the current flow. As a result, the positive half-cycle of the input waveform appears across the load resistor.

During the negative half-cycle of the input waveform, the diode polarities reverse. The diode connected to the bottom half of the secondary winding becomes forward-biased, and the diode connected to the top half becomes reverse-biased. This allows current to flow through the bottom diode and blocks it in the top diode. Therefore, the negative half-cycle of the input waveform also appears across the load resistor.

4. Output Waveform:
The rectification process results in a pulsating DC waveform at the output. This waveform consists of positive half-cycles during the positive half-cycles of the input waveform and negative half-cycles during the negative half-cycles of the input waveform. The output waveform has a frequency twice that of the input waveform since both the positive and negative half-cycles of the input waveform contribute to the output waveform.

5. Ripple Frequency:
The ripple frequency refers to the frequency of the variations or fluctuations in the pulsating DC waveform. Since the output waveform consists of positive and negative half-cycles of the input waveform, the ripple frequency is twice the frequency of the input waveform.

In this case, the input waveform has a frequency of 100Hz, so the output ripple frequency will be 2 * 100Hz = 200Hz.

Therefore, the correct answer is option 'B' - 100Hz.