The sequence 3, 9, 12, 15 follows an alternating pattern: +6, +3, +3, so the next increment should be +6, giving 18. This is a combined arithmetic pattern where the increments alternate between 6 and 3, making it a pattern recognition question rather than a simple arithmetic progression. Option A (17) would come from +2, B (30) from +15, and C (21) from +6 following a different pattern.