Final answer:
The square root of 41 is an irrational number because 41 is not a perfect square and its square root cannot be expressed as a simple fraction.
Step-by-step explanation:
To determine which of the given options is an irrational number, we need to evaluate each of the square root expressions provided. An irrational number is a number that cannot be expressed as a ratio of two integers, and it has a non-repeating, non-terminating decimal expansion.
The square root of 41 is an irrational number because 41 is not a perfect square, which means its square root cannot be expressed as a rational number. The negative square roots of 9, 25, and 64 are not irrational because they are the square roots of perfect squares, which result in the negative integers -3, -5, and -8 respectively, making all of them rational numbers.
Hence, the answer to the question is that the square root of 41 is an irrational number.