Hello.
The LCM is the smallest number that they all have in common when adding it by itself.
The multiples of 5: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50...
The multiples of 10: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100...
The multiples of 15: 15, 30, 45, 60, 75, 90, 105, 120, 135, 150...
The Least Common Multiple is 30.
The answer is C. 30