Tiffany- I just pulled out my copy of the Aria. it’s been years since I played it, but I guessed right about the part you are talking about. It’s the start of the grand arpeggios in the harp, with the organ playing the first theme.
The metronome marking at the start of the piece is an eighth note equals 72. Letter D is the start of the recapitulation and says “a tempo”, referring back to the beginning of the piece. So the tempo there is an eighth equals 72. But that may sound a little frantic with all the notes the harp has to play. I would suggest that you figure out what you can play tempo-wise at D and use that tempo for the beginning of the piece. If it sounds too slow for the beginning, play the beginning slightly faster and then relax the tempo down to what you can do at D. Just try not to have too much of a discrepancy between the two sections.
A word of caution. If you have not had a rehearsal with the organist yet, I think you’ll find that from letter D to the end of the piece it’s almost impossible for you to hear clearly what the organ is doing. You’re going to have to explain to him that he will have to watch you and follow your beat. You have to be positioned so the organist and you can see each other. And at D you will probably have to mark beats with your head as you play. Every time I have ever played this piece, I was never really sure if the organ and I were together in the second half of the piece!