LCD means Least Common Denominator
You have to find the LCM (Least Common Multiple) of denominators of two fractions.
1/2 and 2/4 the LCM of 2 and 4 is 2 so their LCD is also 2.
3/7 and 5/6 the LCM of 7 and 6 is 42 so their LCD is also 42.
The LCM of the denominators is also the LCD of the two fractions.