Let’s examine the relationship between each pair of adjacent terms:

The first term, 6, is divided by 2 to give the second term, 3.
The second term, 3, is multiplied by 4 (2^2) to give the third term, 12.
The third term, 12, is divided by 2 to give the fourth term, 6.
The fourth term, 6, is multiplied by 4 (2^2) to give the fifth term, 24.

From this pattern, we can see that each alternate term is obtained by multiplying the previous term by 4 and then dividing by 2. Applying this pattern to the fifth term, which is 24, we would divide it by 2 to get the missing number:
24 ÷ 2 = 12

Therefore, the missing number in the sequence is 12. The correct answer is (b) 12.