Introduction:
This question is related to electrical power and the expressions used to calculate it. The given options represent different formulas for calculating power in electrical circuits. We need to determine which expression is not a valid representation of power.

Explanation:
Power (P) is the rate at which work is done or energy is transferred. In electrical circuits, power can be calculated using different formulas based on the given variables. Let's analyze each expression given in the options:

a) P = VI:
This expression represents the power (P) in terms of voltage (V) and current (I). It is a valid expression for calculating power in a circuit. Power is directly proportional to both voltage and current.

b) P = I^2R:
This expression represents the power (P) in terms of current (I) and resistance (R). It is known as the Joule's law of heating. According to this law, the power dissipated in a resistive element is directly proportional to the square of the current passing through it and the resistance of the element. This expression is also a valid representation of power.

c) P = V^2/R:
This expression represents the power (P) in terms of voltage (V) and resistance (R). It is derived using Ohm's law (V = IR) and the formula for power in terms of current and resistance (P = I^2R). This expression is a valid representation of power as well.

d) All of the mentioned:
If the correct answer is option 'D', it means that all of the expressions mentioned in the given options are valid representations of power. However, this option contradicts the purpose of the question, which asks for the expression that is not a valid representation of power. Therefore, option 'D' is incorrect.

Conclusion:
Based on the analysis, the correct answer is option 'D'. All of the mentioned expressions (option 'a', 'b', and 'c') are valid representations of power in electrical circuits.