
Shortcut Trick
Check divisibility of each number by prime numbers up to √n.
For 373: √373 ≈ 19.3. Check 2, 3, 5, 7, 11, 13, 17, 19. None divide 373.
For 221: 13 × 17 = 221 (Not prime).
For 437: 19 × 23 = 437 (Not prime).
For 161: 7 × 23 = 161 (Not prime).
∴ The correct answer is 373.

Alternate Method
Given Numbers: 373, 221, 437, 161
Method: A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.
⇒ 161: 161 ÷ 7 = 23. So, 161 is composite.
⇒ 221: 221 ÷ 13 = 17. So, 221 is composite.
⇒ 437: 437 ÷ 19 = 23. So, 437 is composite.
⇒ 373: Testing primes 2, 3, 5, 7, 11, 13, 17, 19... None divide 373 exactly.
⇒ Since 373 has no factors other than 1 and 373, it is a prime number.
∴ The correct answer is 373.

Additional Information
Prime Number Definition
Numbers having exactly two factors (1 and itself). 2 is the only even prime number.
Test for Primality
To check if 'n' is prime, test divisibility by all primes less than or equal to √n.
Co-prime Numbers
Two numbers are co-prime if their Highest Common Factor (HCF) is 1.