Answer:
b
Explanation:
The definition of a rational number is a number that can be expressed as the ratio of two integers, where the denominator is not zero.
Therefore, we need to determine which of the given options can be expressed as the ratio of two integers.
A. $\sqrt{20}$ can be simplified as $\sqrt{4\cdot 5}=2\sqrt{5}$. This is an irrational number because $\sqrt{5}$ is not rational.
B. $\sqrt{25}=5$ is a rational number because it can be expressed as the ratio $5/1$.
C. $\sqrt{42}$ is an irrational number because it cannot be simplified to an integer ratio.
D. $\sqrt{84}$ can be simplified as $\sqrt{4\cdot 21}=2\sqrt{21}$. This is an irrational number because $\sqrt{21}$ is not rational.
Therefore, the only rational number among the given options is $\boxed{\text{B. }\sqrt{ } 25}$.