The correct answer is option 'C' - Magnitude.

Force is a physical quantity that is used to describe the interaction between objects. It can cause objects to accelerate, decelerate, or change direction. The strength of force is expressed by its magnitude, which refers to the size or quantity of the force.

Here's a detailed explanation of the other options and why they are not the correct answers:

a) Weight:
Weight is the force experienced by an object due to gravity. It is the measure of the gravitational force acting on an object's mass. While weight is a measure of force, it is not the same as the strength of force. Weight depends on the mass and the acceleration due to gravity, but the strength of force is a more general term that can encompass other types of forces beyond gravity.

b) Mass:
Mass is a measure of the amount of matter in an object. It is a scalar quantity and is not directly related to the strength of force. However, the mass of an object does affect the force required to accelerate it. According to Newton's second law of motion, the force acting on an object is equal to the mass of the object multiplied by its acceleration. So while mass is not the strength of force, it is related to the force required to produce a certain acceleration.

d) Longitudinal force:
Longitudinal force refers to a force that acts in the same direction as an object's motion. It is a type of force that can cause objects to accelerate or decelerate. However, it is not a measure of the strength of force in general. The magnitude of a longitudinal force would be a measure of its strength, but the term "longitudinal force" itself does not inherently imply the strength of force.

In conclusion, the strength of force is expressed by its magnitude, which refers to the size or quantity of the force. Other factors such as weight, mass, and the direction of force are related to force but do not directly measure its strength.