To determine whether the sum of \( \sqrt{25} \) (which is 5, a rational number) and 18 (also a rational number) will be rational or irrational, we need to examine the numbers involved.
- \( \sqrt{25} = 5 \), which is a rational number.
- 18 is also a rational number.
According to the Closure Property of Addition, the sum of two rational numbers is always a rational number.
Therefore, the correct response is:
Rational, because both addends are rational.