Which one of the following is a scalar quantity?

The correct answer is Mass.

Explanation:
In physics, physical quantities are classified into two main categories based on whether they require a direction to be specified:

1. Scalar Quantities:
A scalar quantity is a physical quantity that has only magnitude (size or amount) and no specific direction.
Examples include mass, distance, speed, energy, and temperature.
Mass represents the quantity of matter contained in a physical body. It depends solely on the amount of matter and does not possess any directional property. Therefore, mass is a scalar quantity.

2. Vector Quantities:
A vector quantity is a physical quantity that requires both magnitude and a specific direction for its complete description.
Let us evaluate the remaining options:

• Gravity (Gravitational Acceleration / Force): Gravity acts downward toward the center of a celestial body, so it has a specific direction and is a vector quantity.
• Momentum: Momentum is defined as the product of mass and velocity ($p=m\×v$). Since velocity has a direction, momentum also has the same direction as velocity, making it a vector quantity.
• Weight: Weight is the gravitational force acting on a mass ($W=m\×g$). It always acts downward towards the center of Earth, which gives it both magnitude and direction, making it a vector quantity.

Thus, among the given choices, Mass is the only scalar quantity.