Rational number?

For a problem like this, you may not know what a rational number is, but you may still be able to solve it just by looking at whatever answer seems to fit with the others the least. Answer choices A, B, C, and D all produce non-integer values when we take their square root, but answer choice E is the exception.

$\sqrt{\frac{64}{49}}$

Becomes:

$\frac{\sqrt{64}}{\sqrt{49}}$

$\frac{8}{7}$

A rational number is any number that can be expressed as the fraction of two integers, and this is the only option that fits the definition. Or, if you don’t know what a rational number is, you can simply see that this is the only answer that produces integer values once we have taken the root, which makes it stand out from the crowd.