You can only have 7 IF statements in a single formula. Use Vlookup instead.
Create a table (could be on another sheet) of your 11 answers and their
corrpesponding letters. For example, if this table is on Sheet5, starting in
cell A1, the Vlookup for cell B10 (which has the answer selected from the
drop down box) might be:
=VLOOKUP(B10,Sheet5!A1:B11,2,FALSE)
Hope this helps,
Hutch