Which of the following numbers is NOT a prime number?

Correct option is C

Given:

Numbers: 1571, 1471, 1271, 1171

Concept Used:

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.

To check if a number is not prime, we will check if it is divisible by any prime number smaller than its square root.

Solution:

(a) 1571:
The square root of 1571 ≈ 39.63. Check divisibility by primes less than 40:

1571 ÷ 2, 3, 5, 7, 11, ..., 37: Not divisible.
1571 is prime.

(b) 1471:
The square root of 1471 ≈ 38.37. Check divisibility by primes less than 38:

1471 ÷ 2, 3, 5, 7, 11, ..., 37: Not divisible.
1471 is prime.

(c) 1271:
The square root of 1271 ≈ 35.65. Check divisibility by primes less than 35:

1271 ÷ 2, 3, 5, 7, 11, ..., 31: divisible by 31.
1271 is not a prime.

(d) 1171:
The square root of 1171 ≈ 34.21. Check divisibility by primes less than 34:

1171 ÷ 2, 3, 5, 7, 11, ..., 31: Not divisible
1171 is a prime.