Find the missing number?

The missing number is 1.

The numbers in the sequence are all prime numbers, except for the missing number. The prime numbers between 1 and 20 are 2, 3, 5, 7, 11, 13, 17, 19, and 23. The missing number is not a prime number, so it must be 1.

Here is a brief explanation of each option:

• Option A: 19 is not a prime number. It is a composite number, which is a number that has more than two factors.
• Option B: 6 is not a prime number. It is a composite number.
• Option C: 17 is a prime number. However, it is not the missing number because the missing number is not a prime number.
• Option D: 1 is the missing number. It is the only number in the list that is not a prime number.