For a number to be in correct scientific notation, the first number or the decimal number has to be less than 10. In this case, 75.3 is not less than 10, hence it is the incorrect answer.
Scientific notation usually writes as c x 10^n, where c is between 1 and 10. The coefficient of B 75.3 is greater than 10, and also is not in the range of 1 and 10, so b is used scientific notation incorrectly.
