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:
$$\text{Percentage decrease}=\left(\frac{0.25x}{1.25x}\right)\times 100$$
$$\text{Percentage decrease}=\left(\frac{0.25}{1.25}\right)\times 100$$
$$\text{Percentage decrease}=0.2\times 100=20\mathrm{\%}$$


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

The correct answer is C. 20%.