The solution of the equation 3d - 6 = 5 is

3d - 6 = 5 --> add 6 to each side 
3d = 11 --> divide by three on each side
D = 3.66666666

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.