There are several information's already given in the question. Using those information's, the answer can be easily deduced.
Sarah's age now = x years
Then
Age of Ralph now = 3x years
After 6 years, the age of Sarah and Ralph will increase by 6 years.
Age of Ralph after 6 years = 3x + 6
So, the correct option among all the options that are given in the question is the last option or option "D". I hope the answer comes to your help.