D the fashion now is to have a drapery rod floating somewhere above the actual window but I like t hem at the top of the trim. The upper windows were not ment to be covered.

Do you need the curtains to fully close or are they just decorative?

If just decorative, we typically install in the "B" range of your photo and we frequently use a short rod on each side instead of a solid rod going all the way across.

For panels, I would do from the D line only because of those upper windows.

I would also think about solar shades instead. Or both. That way the panels can stay open.

Lovely home and view! My preference is to not block the view.

I was thinking about D and keeping the top open

Take the rod above the upper window as TraVerse Enterprises advised (level B). Short rods on either side or one rod over the entire opening will work. Your choice. However placing the rod between the upper and lower windows will make the windows look short and squatty. You’ll literally ruin the beauty and wow factor of having this large wall of windows. What is the top of window to floor measurement for that lower window? It looks very low. Do not place the rod at level C or D.

How about a subtle accent to the whole wall…painting it a pretty pale color?..no curtains…adding rugs and pillows with the same color to keep the interest down at “people level”…with all that natural light …some pretty plants should flourish in this room as well

The plant on the left is a perfect size…but in this setting symmetrical plants will be more suitable

I stand firm on my choice. I wouldn't have a problem with TraVerse’s idea, it is fine all the time, but.,,,,.you have an arched window and that makes the difference.

I see it as a complete design feature and it seems odd to cover part of it.

Here is my thought: You have this amazing window design that also has this even more amazing view, and you want to cover it up with curtains. I would move the chairs around and enjoy the views. I would sit there all day with a hot chocolate and a book or a cognac at night. And for window treatment, I would use inside mount bottom-up top-down shade to manage privacy and light. Inside mount to keep the window design integrity for sure. 3 shades on the bottom and nothing on the top. Light filtering for sure.

I suggest you install full length curtains for a more elegant look and give the room height taking advantage of your high ceiling . You have 2 tiny plant on either side of your armchairs, get larger plants and group them for more impact. Perhaps you're in the process of organizing your room but accessories will contribute to give warmth and a welcoming feel to the room (area rug, accent pillows, throw, etc.).