Explanation: Grade of concrete: As per IS: 456-1978, the different grades of concrete are given as M10, M15, M20, M25, M30, M35, and M40. The letter M denotes the Mix and the numbers 10, 15, 20, 25, 30, 35, 40 represent the predetermined cube strength of 15 cm cube after curing of 28 days in N/mm2. The list of specified concrete grades are given below: Grade of concrete Specified compressive strength at 28 days ( N/mm2) M10 10 M15 15 M20 20 M25 25 M30 30 M35 35 M40 40 In the question, the value of M100, M200, M250, M150, M50, M350, M300 is given in kg/cm2, Not in N/mm2. Hence the correct answer is option 2 for the question. Note: M5 and M7.5 concrete mixes also exist which can be used in lean concrete work and for ordinary masonry wall foundation.