Introduction:
Combinational circuits are digital circuits that produce an output based on the current input values. These circuits do not have any memory elements, and the output is solely determined by the combination of inputs at any given time.

Explanation:
Combinational circuits can be designed using different types of logic gates. However, it is possible to design any combinational circuit using only NOR gates. This can be proven by understanding the properties and functionality of NOR gates.

Properties of NOR gates:
1. NOR gate is a universal gate, which means it can be used to implement any logic function.
2. NOR gate gives an output of logic 0 only when all its inputs are logic 1.
3. The output of a NOR gate is the complement of the OR gate.

Designing combinational circuits using NOR gates:
Using NOR gates, we can implement other basic logic gates such as AND, OR, and XOR gates. Once we have these basic gates, we can combine them to design any desired combinational circuit.

1. Implementing AND gate using NOR gates:
- Connect the inputs of the NOR gate to each other.
- Connect the output of the NOR gate to another NOR gate.
- Connect the output of the second NOR gate to its own input.
- The output of the second NOR gate will be the AND function.

2. Implementing OR gate using NOR gates:
- Connect the inputs of the NOR gate to each other.
- The output of the NOR gate will be the complement of the OR function.

3. Implementing XOR gate using NOR gates:
- Connect the inputs of the NOR gate to each other.
- Connect the inputs to another NOR gate.
- Connect the output of the second NOR gate to its own input.
- The output of the second NOR gate will be the XOR function.

Conclusion:
Since any combinational circuit can be designed using only NOR gates, option D - NOR gates is the correct answer. NOR gates are versatile and can be used to implement any logic function, making them a powerful tool in digital circuit design.