Which property is primarily modified by adding a resistor to a circuit?

Adding a resistor to a circuit primarily modifies the resistance of that circuit. When a resistor is introduced, it provides a specific opposition to the flow of electric current. This change in resistance can significantly influence how the circuit operates, including altering the flow of current and affecting voltage levels across other components, following Ohm's Law (V = IR).

By increasing the overall resistance in the circuit, the resistor can reduce the overall current flowing for a given voltage. Therefore, the addition of a resistor is a direct modification of the resistance property, which in turn impacts other properties like current and voltage, but it is the resistance that is specifically modified by the presence of the resistor.