Which of these is NOT an example of a reference direction?

A. +

B. S

C. down

D. 2 E

D is the answer obviously

So we can easily tell that the answer is D. So the + means positive or in the positive side of the graph in the y or x. So that also references direction. The S also stands for South in a compass. C is of course direction down is a direction. That only leaves D which is 2 E or 2e which is a number not a direction. 2e means 2 times euler's number.

Answer: D