House from the 50's.
Not much of a plumber but just looking, seems a lot of hacking over time.
What is item B? What's it for if name is not self descriptive? Nut on side - a drain? Nut on top?
D goes to a toilet
C goes no place. It's capped off out of pic.
A is a Beacon Mixing Valve 69. There's an adjustment screw on the other side. Web says: "To operate: by turning the adjusting knob to the right (clockwise) you can increase the water temperature; to lower the water temperature, turn the knob to the left (counterclockwise)"
Don't know what there is a mixing valve there for. It's all cold water. Guessing it had a former use.
Unless B has a current function I'm guessing the whole mess from B to right could be removed and 1/2" Cu direct to toilet????