D Deck was a location aboard the city-sized spaceship Red Dwarf.
When Rimmer was briefly promoted to an officer, he had the skutters build a new Officer's Club on D Deck, which was out of bounds to everybody else. He filled it with bio-printed clones of himself, although this did not end well. ("Officer Rimmer", Series XI)
When Kryten fixed an external tracking sensor on D Deck, much to everyone else's disappointment it located Lister's Guitar, which had been floating in space for a year or so. ("Siliconia", Series XII)