The correct answer is: B. M20
The minimum grade of concrete to be used in reinforced concrete as per IS: 456-1978 is M20. This grade of concrete has a compressive strength of 20 MPa. It is a good all-purpose grade of concrete that can be used for a variety of applications, including residential and commercial buildings, bridges, and other structures.
The other options are incorrect because they do not meet the minimum grade requirements specified in IS: 456-1978. Option A, M15, has a compressive strength of 15 MPa. Option C, M10, has a compressive strength of 10 MPa. Option D, M25, has a compressive strength of 25 MPa.
IS: 456-1978 is the Indian Standard Code of Practice for Plain and Reinforced Concrete. It is a comprehensive code that covers all aspects of the design and construction of concrete structures. The code is updated periodically to reflect the latest developments in concrete technology.