I have been using agenda on my Macbook and iPad, with the option store documents and data on iCloud. Now I want to shift this to Dropbox.
While the settings on Macbook give the option of selecting Dropbox. On iPad I am not able to getting the Dropbox option. Only iCloud drive and On My iPad are appearing
Perhaps you are looking in the wrong place. You need to look in the Agenda app itself.
Make sure you have the Dropbox app installed on your iPad, and are signed in
Open the right panel
Tap the button bottom-left for settings
There is an option there for sync. Tap that, and set to Dropbox.
Use Dropbox app to sign in
This will sync your Agenda via Dropbox. Note that it will not make your notes editable in Dropbox as files though. The data stored there is not human readable, and is only for sync purposes.