Line D is the fourth line in the Splatoon 2: Octo Expansion DLC. It contains eleven stations, but does not contain any “Thangs”.
Line D is a grey-coded line that’s a mostly horizontal line with the slight curve down at the end. This line intersects with Line B (cyan) at the far left at D01. Ink from Above Station, and to the right it meets Line H (dark blue) at D03. Slap Bracelet Station. On the far right it intersects with Line J (pink) in two places, at D08. Girl Power Station, and on the bottom right at D11. Ink and Watch Station.
See the list of Stations below for full walkthroughs: