Number of prime numbers from 1 to 50 are: (1) 18     (2) 12      (3) 15       (4) 20

Any whole number greater than 1 that is divisible only by 1 and itself, is defined as a prime number.

Answer: The number of prime numbers from 1 to 50 is option(3) - 15. 

Let's explore prime numbers in detail.

Explanation:

A prime number is a number greater than 1 that has exactly two factors.

Example: 3 can be factorized in only one way, that is, 1 × 3 (OR) 3 × 1. It has only two factors, which are, 1 and 3. Therefore, 3 is a prime number.

With the help of the Sieve of Eratosthenes, we can find the primes numbers up to a given number. The primes from 1 to 50 are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, and 47.

Thus, there are 15 prime numbers from 1 to 50.