Ohm’s law expressed as $E = IR$.
A. Applied to ac when circuit is purely resistive.
B. Applies to ac in the same manner as dc do.
C. Always applies to a circuit when $Z$ is substituted for $R$.
D. Tell us that, ${E_{eff}} = 0.707\left( {{E_{\max }}} \right)$ for ac.

Hint: By gathering, all the inspirations of the previous scientists who worked on the resistance and the related theories, ohm’s law is formulated by ohm. The ohm’s law is basically a description of electricity and how the electricity works. Ohm’s law is also used for calculating the value of resistance, the current and the voltage.

Complete step by step answer:
The ohm’s law is derived by the scientist ohm by focusing on the experiment given below:
In this experiment, ohm used a thermocouple that is known as a stable source of voltage. To measure this voltage, he had used a galvanometer. He was aware that the temperature of the junction is directly proportional to the voltage across the two terminals of the thermocouple.

To make the circuit, used in this experiment, a closed circuit, he used various wires of varying length, properties, and diameter. The result of this experiment is derived from the equation given below
$E = IR$
Here, $E$ is the emf of the thermocouple, $I$ is the current in the circuit and $R$ is the resistance in the circuit.
This equation always applies to a circuit when the value of $Z$ is substituted into $R$.

Hence, option C is the correct option.

Note:Here, $Z$ is the impedance in the circuit and $R$ is the resistance in the circuit. When the value of $Z$ is substituted into $R$, the value of impedance will become the same as that of the resistance. This is the case, when ${X_L} = {X_C}$. Here, ${X_L}$ is known as an inductive reactance and ${X_C}$ is known as a capacitive reactance.