The correct option is A 12
The smallest number that is a common multiple of any 2 or more numbers is the least common multiple (LCM) of those numbers.
Multiples of 4: 4, 8, 12, 16, 20, 24, 28, 32, 36, 40 ......
Multiples of 6: 6, 12, 18, 24, 30, 36, 42, 48, 54, 60 .........
Multiples of 12 : 12, 24, 36, 48, 60, 72, 84, 96, 108, 120...
As you can see, 12 is the smallest multiple that is common to all the three numbers.
So, the LCM of 4, 6, and 12 is 12.