If A's salary is 25% more than B's salary, then B's salary is how much lower than A's salary?

Answer: Option C

Let B's Salary is Rs. 100. Then,
A's Salary = (100 + 25% of 100) = Rs. 125
Difference between A's Salary and B's Salary = 125 - 100 = Rs. 25
% Difference (lower) = $$\frac{25}{125}\times 100=20\%$$

Mind Calculation Method:
100(B salary) === 25%↑ ===> 125(A salary) === 20%↓ ===> 100 (B salary)
B's salary is 20% lower than A's