Introduction:
A practical current source is a device that provides a constant current regardless of the load connected to it. It is commonly used in electronic circuits to drive LEDs, transistors, and other devices that require a constant current for proper operation. In order to represent a practical current source, it is necessary to combine it with other circuit elements, such as resistors, to account for the characteristics and limitations of the source.

Explanation:
The correct representation of a practical current source is a resistance in parallel with an ideal current source. This means that the practical current source is connected in parallel with a resistor. Let's understand why this representation is appropriate:

1. Ideal current source:
An ideal current source is a theoretical concept that provides a constant current regardless of the load or external factors. It is represented by a short circuit symbol in circuit diagrams. However, in practical applications, it is not possible to achieve a perfect constant current source.

2. Resistance in parallel:
By connecting a resistance in parallel with the ideal current source, we can account for the limitations of the practical current source. The resistance represents the internal resistance of the source and the resistance of the load connected to it. When a load is connected to the current source, the voltage across the resistance causes a drop in the current. This drop can be modeled by the resistor in parallel.

3. Equivalent circuit:
The combination of the ideal current source and the resistance in parallel forms an equivalent circuit that accurately represents the behavior of the practical current source. This representation allows us to analyze the circuit and calculate the current flowing through the load based on the voltage across the resistance.

4. Alternative representations:
The other options mentioned in the question (resistance in series with an ideal current source, resistance in parallel with an ideal voltage source, and none of the above) do not accurately represent the behavior of a practical current source. A resistance in series with an ideal current source would cause a drop in the current, which is not characteristic of a constant current source. A resistance in parallel with an ideal voltage source would result in a variable current depending on the load connected to the source. None of the above options accurately represent the behavior of a practical current source.

Conclusion:
In conclusion, a practical current source is represented by a resistance in parallel with an ideal current source. This representation accurately models the behavior of the practical current source, taking into account the internal resistance and the load connected to it.