Identify the Odd One Out:

Looking at each number, let's examine their prime factorizations:

$175 = 5^2 \times 7 = 25 \times 7$

Prime factors: 5, 7 (both single-digit primes)

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$137$ is a prime number.

To verify: $\sqrt{137} \approx 11.7$

Checking divisibility by primes up to 11: not divisible by 2, 3, 5, 7, or 11.

Therefore, 137 is prime (can be considered as having itself as the only prime factor).

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$247 = 13 \times 19$

Prime factors: 13, 19 (both double-digit primes)

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$215 = 5 \times 43$

Prime factors: 5, 43 (one single-digit prime)

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The pattern:

$175$: Contains single-digit prime factor (5, 7)

$137$: Prime number itself (single-digit prime when considering it has only itself)

$215$: Contains single-digit prime factor (5)

$247$: All prime factors are double-digit (13 and 19)

Therefore, 247 is the odd one out as it is the only number whose prime factorization contains exclusively double-digit primes.