Explanation:

The given series is 6, 6, 12, 36, 144, 720, ?

6

6 x 1 = 6

6 x 2 = 12

12 x 3 = 36

36 x 4 = 144

144 x 5 = 720

720 x 6 = 4320

Hence, the next number in the given number series is 4320.

Hence, option 1 is the correct answer.