Which of the following means that an image is located behind a lens? a. +di b. -di c. -do d. +do

For a lens, the following sign convention is generally used:

- f (the focal length) is positive for a converging lens and negative for a diverging lens
-  (the distance of the object from the lens) is positive if the object is in front of the lens and negative if it is behind the lens
-  (distance of the image from the lens) is positive if the image is behind the lens (real image) and negative if the image is in front of the lens (virtual image)

Therefore, the correct option is 
A.+di
which mens that the image is real and located behind the lens.