Identify the Odd One Out:

To identify the odd one out, we need to examine the properties of each number.

Checking if each number is prime:

For 191:

Testing divisibility by primes up to $\sqrt{191} \approx 13.8$

191 is not divisible by 2, 3, 5, 7, 11, or 13

Therefore, 191 is prime.

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For 149:

Testing divisibility by primes up to $\sqrt{149} \approx 12.2$

149 is not divisible by 2, 3, 5, 7, or 11

Therefore, 149 is prime.

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For 273:

Testing divisibility: $273 \div 3 = 91$

$273 = 3 \times 91$

$273 = 3 \times 7 \times 13$

Therefore, 273 is composite (not prime).

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For 233:

Testing divisibility by primes up to $\sqrt{233} \approx 15.3$

233 is not divisible by 2, 3, 5, 7, 11, or 13

Therefore, 233 is prime.

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The numbers 191, 149, and 233 are all prime numbers.

The number 273 is a composite number.

Therefore, 273 is the odd one out.