There are several ways to go about this.
You can inform the airline in advance that you intend to terminate at point D. By doing this, however, they may demand that you modify your ticket to terminate at D. This in turn may well result in a higher fare and/or a change fee, strange as it sounds; such is commercial aviation.
You can simply no-show at point D. ...
Airlines generally don't like this, because it's often used for Hidden City Ticketing, in which you buy a ticket A-B-C intending to only go to B, because it's cheaper than the fare for A-B.
The answers to this question cover the potential issues, but in short:
It's probably fine as a one-off.
Be aware that the airline is obliged to get you to A, not D, so ...