To find the lowest common denominator between the fractions, what you need to do is simply write out the multiples of both 3 and 5 and find what multiple is common between both:
3,6,9,12,15 5,10,15
The multiple that is common and is the lowest in both would be 15.
The solution thus is B.15, it is the LCD for the fractions of 2/3 and 4/5.
