Which of the following is a prime number? 

Concept -

A prime number is a positive integer greater than 1 that has exactly two distinct positive divisors: 1 and itself.

In other words, a prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers other than 1 and itself.

For example, 2 is the first prime number because its only divisors are 1 and 2. Other examples of prime numbers include 3, 5, 7, 11, 13, 17, and so on.

Explanation -

For option(1) -

15 = 3 x 5

For option(2) -

23 = 23 x 1

For option(3) -

12 = 3 x 4 

For option(4) -

75 = 15 x 5

Hence the correct answer is 23.