What is the least common denominator (LCD) of 1/2 and 2/3 ?A. 5 B. 6C. 12 D. 18

So basically you just want to find the LCM of the denominators shown, or 2 and 3. 

So in this case, you could use the most inefficient method, or listing the multiples out:

2 : 2, 4, 6

3: 3, 6, 9

And as you can see, the first multiple that matches is 6. So your answer would be B, 6.