Final answer:
To use the superposition theorem, you should analyze one source at a time and sum their contributions. The direction of current flow can be assumed, with incorrect assumptions resulting in negative current values that suggest the opposite direction.
Step-by-step explanation:
To use the superposition theorem, option C is correct: analyze one source at a time. This principle is important for analyzing circuits with more than one source where other methods, such as Ohm's law or series-parallel analysis, are not applicable. Instead, you address each source in isolation, turn other sources off by replacing them with their internal resistance (voltage sources become short circuits, and current sources become open circuits), and then calculate the effects on the circuit. After analyzing each source separately, you sum the contributions of each to determine the total current or voltage at any point in the circuit.
As for the direction of current flow, you should note that while determining the direction at the start is helpful, if the direction chosen is incorrect, the final current values obtained will have a negative sign, indicating the true direction is opposite to what was assumed. For example, if the current in the branch with the voltage source is upward and currents in the other two branches are downward, such that Ia = I₁ + Ic, relevant current values will satisfy the junction rule.