Explanation:

The frequency of ripple in the output voltage of a three-phase controlled bridge rectifier depends on various factors. Let's discuss each option to understand which one affects the frequency of ripple.

Firing Angle:
The firing angle determines when the thyristors in the bridge rectifier start conducting. It controls the amount of power delivered to the load. However, the firing angle does not directly affect the frequency of ripple. It only affects the magnitude of ripple voltage and the average output voltage.

Load Inductance:
The load inductance plays a crucial role in smoothing out the output voltage of the rectifier. It reduces the ripple component by storing energy during the conduction period of the thyristors and releasing it during the non-conduction period. However, the presence of load inductance does not affect the frequency of the ripple voltage.

Load Resistance:
The load resistance determines the current flowing through the load. It affects the magnitude of ripple voltage but has no impact on the frequency of ripple.

Supply Frequency:
The supply frequency is the frequency at which the input AC voltage is supplied to the bridge rectifier. It directly affects the frequency of the ripple voltage. The ripple frequency is given by the formula:

Ripple frequency = (3 × supply frequency) ± (3 × harmonic frequency)

The harmonic frequency is dependent on the number of pulses generated by the bridge rectifier. In a three-phase controlled bridge rectifier, there are six thyristors, resulting in six pulses per cycle. Therefore, the harmonic frequency is six times the supply frequency.

So, the frequency of the ripple voltage is directly proportional to the supply frequency. If the supply frequency increases, the ripple frequency also increases, and vice versa.

Hence, the correct answer is option 'D' - supply frequency.