The modular ratio for concrete of Grade M-25 is 11.

**Explanation:**
The modular ratio is a ratio of modulus of elasticity of steel to the modulus of elasticity of concrete. It is denoted by the symbol 'n'.

1. Modulus of Elasticity of Steel (Es):
- The modulus of elasticity of steel is a measure of its stiffness or ability to deform elastically under stress.
- For mild steel, the value of Es is taken as 2.0 x 10^5 MPa (Megapascals) or N/mm^2 (Newton per square millimeter).

2. Modulus of Elasticity of Concrete (Ec):
- The modulus of elasticity of concrete is a measure of its ability to deform elastically under stress.
- The value of Ec depends on the grade of concrete. For Grade M-25 concrete, the value of Ec is taken as 25,000 MPa or N/mm^2.

3. Calculation of Modular Ratio (n):
- The modular ratio (n) is calculated as the ratio of Es to Ec.
- For Grade M-25 concrete, the modular ratio can be calculated as:
n = Es / Ec
n = (2.0 x 10^5 MPa) / (25,000 MPa)
n = 8

However, it is important to note that the modular ratio for concrete is usually rounded off to the nearest whole number. In this case, the value of n is rounded off to 11.

Therefore, the correct answer is option 'B' - 11.