This should be easy but I am stuck and would welcome some help or
clues.

I have a macro which does various processes. In the middle of the
macro , I need to pause, ask the user to select a particular worksheet
from a list of 20. The macro continues, pasting a range to the
selected worksheet.

At present I simply do this using a userform and the user types in the
worksheet name from the list of 20 but this is prone to user typing
error. I would prefer a drop down list, ideally showing only the
remaining choices (so that after 18 iterations, there are only the
last two sheet names left).

Here is what I have at present:
Worksheets("Index").Activate
SelectAnswer = InputBox("Type in - carefully! the worksheet name from
this list.")
On Error Resume Next
Worksheets(SelectAnswer).Activate
Range("A1").Select
ActiveSheet.Paste