Answer: 5 and 12 
Step-by-step explanation:
2 and 12 is not it   because 12 can be divided by 2 meaning that the LCM of those two numbers will be 12. 
6 and 12 is not it because  12 can also be divided  by 6 meaning that the LCM of 6 and 12 is also 12.
3 and 12 is also the same,  12  can be divided by 3 so their LCM will be 12. 
With 5 and 12, 12 is not divisible by 5 so  you will have to find the multiples of 5 and 12 and it will eventually be 60. 
Multiples of 5:  5, 10,15,20,25,30,35,40,45,50,55,60,65 
Multiples of 12:  12, 24, 36, 48, 60, 72  
As it appears in both multiples, 60 is the least number that appears in both multiples so the LCM of 5 and 12 is 60.