7, 10, 8, 11, 9, 12, ... What number should come next?

Answer: Option B

This is a simple alternating addition and subtraction series. In the first pattern, 3 is added; in the second, 2 is subtracted.

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Alternate Solution :
Above Series contains two sub series in it
Series 1^st = 7, 8, 9, 10
Series 2^nd = 10, 11, 12

Note : Both sub series is in ascending order. Next number is greater by 1 from the previous one.