Stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.

**Stress can be defined as the magnitude of Internal Resisting Force (IRF)**

Stress is a term commonly used in the field of mechanics and materials science. It refers to the internal forces that develop within a material when it is subjected to external loads or deformations. Stress can be defined as the magnitude of the Internal Resisting Force (IRF) experienced by a material per unit area.

**Understanding Internal Resisting Force (IRF)**

Internal Resisting Force (IRF) is the force that a material exerts on itself to resist deformation or failure. When an external load is applied to a material, it causes the material to deform. This deformation leads to the development of internal forces within the material, which resist the deformation. These internal forces are known as the Internal Resisting Force.

**Magnitude of IRF as a measure of Stress**

The magnitude of the Internal Resisting Force (IRF) is a measure of the stress experienced by a material. It represents the amount of force per unit area that the material is resisting against due to the applied load. In other words, stress is the intensity of the internal forces within the material.

**Importance of Cross-Sectional Area and Surface Area**

While the cross-sectional area and surface area of a material are important factors in determining the overall strength and behavior of a material, they are not directly related to the definition of stress. Cross-sectional area refers to the area of a plane that is perpendicular to the direction of the applied load. Surface area, on the other hand, refers to the total area of the external surface of a material.

**Correct Answer - Magnitude of IRF**

The correct answer to the given question is option 'B', which states that stress can be defined as the magnitude of the Internal Resisting Force (IRF). This definition aligns with the fundamental understanding of stress as the intensity of the internal forces within a material. Stress is measured in units of force per unit area, such as Pascals (Pa) or pounds per square inch (psi).

In conclusion, stress is defined as the magnitude of the Internal Resisting Force (IRF) experienced by a material per unit area. While the cross-sectional area and surface area of a material are important factors, they are not directly related to the definition of stress. It is essential to understand the fundamental concept of stress and its relationship with the internal forces within a material to analyze and design structures and components effectively.