Introduction:
A digital-to-analog converter (DAC) is a device that converts digital signals into analog signals. The input DAC converts digital input values into corresponding analog voltage levels. The number of discrete voltage levels that an input DAC can generate determines its resolution.

Explanation:
The given question states that an 8-input DAC has a certain number of discrete voltage levels. Let's analyze each option to determine the correct answer.

a) 8 discrete voltage levels:
If the DAC had only 8 discrete voltage levels, it means it would only be able to generate 8 different output voltage values. However, an 8-input DAC implies that it can accept 8 bits of digital input. Each input bit can have two possible values: 0 or 1. Therefore, the total number of possible combinations of the 8 input bits is 2^8 = 256. Hence, option 'a' is incorrect.

b) 64 discrete voltage levels:
Similarly, if the DAC had 64 discrete voltage levels, it would imply that it can accept 6 bits of digital input (2^6 = 64). However, the given DAC is an 8-input DAC, so it can accept 8 bits of digital input. Hence, option 'b' is incorrect.

c) 124 discrete voltage levels:
This option also suggests that the DAC can generate 124 discrete voltage levels. However, this value does not align with the number of input bits (8 bits) provided in the question. Therefore, option 'c' is incorrect.

d) 256 discrete voltage levels:
This option states that the DAC can generate 256 discrete voltage levels. Since the DAC has 8 inputs, each input bit contributes to the total number of possible combinations. As mentioned earlier, the total number of combinations for 8 bits is 2^8 = 256. Hence, option 'd' is correct.

Conclusion:
Based on the analysis, option 'd' is the correct answer. An 8-input DAC can generate 256 discrete voltage levels, corresponding to the 8 bits of digital input it can accept.