So, let us try to explain all the options in order...

Scalar quantities are physical quantities that have only magnitude and no direction. They can be completely described by a single numerical value. On the other hand, vector quantities have both magnitude and direction.

Out of the given options, the correct answer is option 'D' - current. Let's understand why current is a scalar quantity.

Electric current is the rate at which electric charges flow through a conductor. It is given by the equation I = Q/t, where I is the current, Q is the charge flowing through the conductor, and t is the time taken. The unit of electric current is the ampere (A).

Explanation:
1. Linear Momentum:
Linear momentum is the product of an object's mass and its velocity. It is a vector quantity as it depends on both magnitude and direction. Therefore, option 'A' is incorrect.

2. Relative Velocity:
Relative velocity is the velocity of an object with respect to another object. It is also a vector quantity as it depends on both magnitude and direction. Therefore, option 'B' is incorrect.

3. Average Velocity:
Average velocity is the displacement of an object divided by the time taken. It is also a vector quantity as it depends on both magnitude and direction. Therefore, option 'C' is incorrect.

4. Current:
Electric current is the rate at which electric charges flow through a conductor. It is a scalar quantity as it only has magnitude and no direction. Current is measured in amperes, and its direction is determined by the flow of positive charges. However, the magnitude of current does not depend on the direction of flow. Therefore, option 'D' is the correct answer.

In conclusion, out of the given options, only electric current (option 'D') is a scalar quantity.