To find out by what percent B's salary is lower than A's salary, follow these steps:

1. Define the salaries:
- Let \( B \)'s salary be \( x \).
- A's salary is 25% higher than B's, so A's salary is \( x + 0.25x = 1.25x \).

2. Calculate the difference between A's and B's salaries:
\[
\text{Difference} = 1.25x - x = 0.25x
\]

3. Calculate the percentage difference relative to A's salary:
To find the percentage by which B's salary is lower than A's salary, use the formula:
\[
\text{Percentage decrease} = \left(\frac{\text{Difference}}{\text{A's salary}}\right) \times 100
\]
Substitute the values:
\[
\text{Percentage decrease} = \left(\frac{0.25x}{1.25x}\right) \times 100
\]
\[
\text{Percentage decrease} = \left(\frac{0.25}{1.25}\right) \times 100
\]
\[
\text{Percentage decrease} = 0.2 \times 100 = 20\%
\]


B's salary is 20% lower than A's salary.

The correct answer is C. 20%.