The differences are: 15-6=9, 35-15=20, 77-35=42, 143-77=66. These differences (9, 20, 42, 66) are approximately 2×previous: 2×9=18≈20, 2×20=40≈42, 2×42=84≠66. Alternative: each term = 2×previous + increasing offset: 6×2+3=15, 15×2+5=35, 35×2+7=77, 77×2+9=163 (not 143). Let me verify: 143 = 77 + 66. If differences follow pattern 9, 20, 42, 66 - the second differences are 11, 22, 24. Pattern unclear but answer B (221) matches 143 + 78.