The superposition theorem is a powerful technique used in electrical circuit analysis to simplify complex circuits and calculate voltages and currents. It states that the total response in a linear circuit with multiple sources can be determined by considering the individual contributions of each source separately.

The superposition theorem requires as many circuits to be solved as there are sources. This means that for a circuit with multiple sources, each source is analyzed individually while the other sources are turned off. The responses from each source are then combined to determine the overall response of the circuit.

Here is a detailed explanation of why the correct answer is option 'D' - sources:

1. Understanding the concept of superposition:
- The superposition theorem is based on the principle of linearity, which states that the response of a linear circuit to a sum of inputs is equal to the sum of the responses to each individual input.
- In other words, if we have multiple sources in a circuit, we can find the response caused by each individual source and then add them to get the overall response.
- This is done by turning off all other sources except the one being considered at a time.

2. Analyzing the circuit for each source:
- To apply the superposition theorem, we need to solve the circuit for each source individually.
- This means that we turn off all other independent sources (voltage or current sources) and consider only one source at a time.
- By doing this, we can calculate the voltage or current caused by that particular source.

3. Combining the individual responses:
- Once we have calculated the voltage or current for each source, we can combine them to find the total response of the circuit.
- This can be done by algebraically adding the voltages or currents obtained from each source.

4. Why nodes and meshes are not considered for solving the circuit:
- While nodes and meshes are important concepts in circuit analysis, they are not directly related to the application of the superposition theorem.
- Nodes are points in a circuit where multiple elements are connected, and meshes are loops formed by interconnected elements.
- The superposition theorem focuses on analyzing the effects of individual sources, rather than the specific characteristics of nodes and meshes.

In conclusion, the superposition theorem requires as many circuits to be solved as there are sources in the circuit. This is because each source needs to be considered individually while all other sources are turned off. Nodes and meshes are not directly related to the application of the superposition theorem.