Correct Answer:
C. √21
A surd is an irrational number that cannot be simplified to remove the square root (or other root) sign. It represents a root of an integer that cannot be expressed as a whole number or a simple fraction.
- √21 is the correct answer because 21 is not a perfect square, and thus √21 cannot be simplified into a rational number.
- √16 simplifies to 4, which is a rational number, not a surd.
- √16 + √25 simplifies to 4 + 5 = 9, which is also a rational number.
Therefore, √21 is the only option that fits the definition of a surd.