Introduction:
A centre tapped full wave rectifier is a type of rectifier circuit that converts alternating current (AC) into direct current (DC). It uses a center-tapped transformer and two diodes to achieve this conversion. When the input AC signal is applied to the primary winding of the transformer, the secondary winding produces two equal and opposite voltage waveforms with respect to the center tap. The diodes then rectify these two waveforms to produce a pulsating DC waveform at the output.

Harmonics in the Output:
Harmonics are multiples of the fundamental frequency present in the output waveform. In the case of a full wave rectifier, the output waveform consists of only even harmonics. This is because the diodes conduct during alternate half cycles of the input AC waveform.

Explanation:
When the positive half cycle of the input AC waveform is applied to the transformer, the upper diode conducts and allows the current to flow through the load. At the same time, the lower diode is reverse biased and blocks the current flow. This results in a positive half cycle at the output.

During the negative half cycle of the input AC waveform, the upper diode is reverse biased and blocks the current flow. However, the lower diode conducts and allows the current to flow through the load in the opposite direction. This results in a negative half cycle at the output.

Even Harmonics:
Since the diodes conduct during alternate half cycles, the output waveform contains only even harmonics. The fundamental frequency is the same as the input AC frequency, and the second harmonic is twice the fundamental frequency. Subsequent even harmonics are also present, each with a frequency that is a multiple of the fundamental frequency.

Odd Harmonics:
In a full wave rectifier, the diodes do not conduct during the same half cycles of the input AC waveform. As a result, odd harmonics are not present in the output waveform.

Conclusion:
In a centre tapped full wave rectifier, the output waveform contains only even harmonics. This is because the diodes conduct during alternate half cycles and do not allow odd harmonics to pass through. Therefore, option 'A' is the correct answer.