The series 3, 5, 7, 11, 13, 15, 19, 23 is meant to follow one rule. Which number is out of place?

Explanation

1. 01

   The terms are the odd primes in order: 3, 5, 7, 11, 13, then 17, 19, 23.

2. 02

   15 is not prime $(15 = 3 \times 5)$; the prime between 13 and 19 is 17.

3. 03

   So 15 is the wrong term; it should be 17.