The correct answer is option 'C' - the modular ratio of two materials is the ratio of their modulus of elasticities.

Explanation:
Modular ratio is a term commonly used in engineering mechanics and materials science to compare the stiffness or rigidity of two different materials. It is defined as the ratio of the modulus of elasticity of one material to the modulus of elasticity of the other material.

The modulus of elasticity, also known as Young's modulus, is a measure of a material's stiffness or ability to resist deformation under an applied load. It quantifies the relationship between stress and strain in a material. When a material is subjected to an external force or stress, it undergoes deformation or strain. The modulus of elasticity relates the stress to the resulting strain in the material.

In the context of the question, the modular ratio compares the stiffness or rigidity of two materials. It tells us how much stiffer or more rigid one material is compared to the other.

For example, let's consider two materials A and B with modulus of elasticities E_A and E_B, respectively. The modular ratio R_AB can be calculated as:

R_AB = E_A / E_B

If the modulus of elasticity of material A is higher than that of material B, the modular ratio R_AB will be greater than 1, indicating that material A is stiffer or more rigid than material B. On the other hand, if the modulus of elasticity of material A is lower than that of material B, the modular ratio R_AB will be less than 1, indicating that material A is less stiff or less rigid than material B.

In summary, the modular ratio of two materials is a measure of their relative stiffness or rigidity, and it is defined as the ratio of their modulus of elasticities.