My thought indeed was that maybe it could be called a D major 7 sus9 because that really points out the 3 important aspects of the chord: 1. That it's a sus chord, so no 3rd (F#in this case) should be played. 2. That it's a major 7th chord, which is very important and distinguishes it from a dominant 7th chord. 3. The sus is played on top of the chord, giving it a very different sound.

All that said I don't remember ever seeing a major 7 sus9 chord.

I definitely didn't mean to imply it was a D major 9 chord. That would indicate a D major 7th chord with the 9th on top (and, of course include the 3rd).

Regrading the last question, I think calling a chord D sus9 would indicate there is no 7th at all (neither major 7th nor dominant 7th). The chord would be spelled D, A, C#, E.

When a 7th is called for in a sus chord, it's written as 7sus4. So if I follow my own logic here, a D dominant sus chord with an E on top might be called a D 7sus9. Again, I've never seen this.

My bottom line on the original quiz question is to call the chord D major 7 sus9. This is unambiguous and tells the musician how to play this chord with the unusual sound that makes "A Horse With No Name" such a unique song.

But that's just me talking.