Rational Numbers

A rational number has a terminating decimal if and only if the denominator, when reduced to its simplest form, contains no prime factors other than 2 and 5. Let's analyze each option:
- : The prime factorization of 250 is , which contains only 2 and 5. Therefore, this number has a terminating decimal.
- : The prime factorization of 225 is , so it has a factor of 3, making it a non-terminating decimal.
- : The prime factorization of 18 is , and since 3 is involved, it will not terminate.
- : The prime factorization of 21 is , so it also does not terminate.
Thus, the correct answer is .