What is this pipe?

Attached is a picture of a basement floor with bathroom rough-in. The plumber that did the work is no longer in business so I can't ask him for an answer.

What is item "D"?

A - is for sink and includes vent
B - 4" line for toilet (even though it is roughed in only 11" from wall.
C - I think is for the shower, but it is positioned in the plane of the wall

D - Is the item in question. It is 5" thin-wall and is plugged about 6" below the level of the floor.

E - Is a 3" drain for who knows what?

I know that I am going to have to excavate the concrete and move the shower drain and toilet, but I need to know if I can cap and cover the item in question (D)

F - is a cleanout and stack vent

This house is on city sewer.

Thanks for the help.

pipes

The locations of the pipes mean nothing. How the pipes are connected together under the floor will dictate what their purpose is. That could be a "sleeve" for a bathtub trap, or whatever else the installer intended it for. A pipe in a wall is usually a vent NOT a drain unless it has a fixture connection in it.

New construction? If so D may be a radon vent.

That seems to be the general consensus of opinion. This is about 10 years old. Most new construction that I have run into has one of these. The thing that is confusing is that it is in the middle of the floor. I would think that for the convenience of the homeowner, the plumber would place it closer to the wall where you would evenually place a radon pump.
Thanks for the response.

an uneducated guess

In my town D could be an access for a backwater valve on the soil pipe. It would be close -- but maybe not THAT close -- to the riser.