Question

6 less than a number is 15. Let x represent the number. Which equation represents this?

Answer 1

**Note / Protip**
Always substitute "a number" for your variable when reading worded equations!
With that being said...

D. x - 6 = 15

Is your answer, since you are going to want to reverse it because of how the problem's worded, not c. because; c.'s worded equation would be
"A number less than six."