Find the missing number?

The missing number is 5.

The numbers in the sequence are all prime numbers, except for 1, which is not a prime number. The sequence starts with 2, which is the first prime number, and then adds 3 to get 5, which is the next prime number. The sequence continues in this way, adding 3 to each number to get the next number in the sequence.

Option A is incorrect because 6 is not a prime number.

Option B is incorrect because 1 is not a prime number.

Option C is incorrect because 8 is not a prime number.

Option D is correct because 5 is a prime number and it is the missing number in the sequence.