These were difficult. Don't feel bad if you guessed that the "D" was added. In the 1970s, one of the top authenticators (in his own mind) was telling everyone that these GENUINE 1932-D 25c had an added mintmark. The discoloration around the "D" makes it even more suspicious. Many coins of this date have "ejection doubled" mint marks. The "O" has been added. It is a pretty good job except the crook left too much discoloration around the "O." It should have been blended in more. There are Morgan dollar collectors who can tell this is not an "O" Mint coin just by its rim!
I voted for the "O" because I don't like the smooth area with a different color around it. I knew that the "D" was on a 1932-D quarter. It's an odd mint mark, and I've only handed one of these coins in my life. Washington Quarters have never been among my favorites. I bought a 1932-D quarter in a raw set I bought as a dealer, and it was real. That is only one I have handled or spent much time with it.
Sorry I’m late to the party but when I opened the thread I noticed that the O looked added, while the D had ejection doubling / die polish (a little more severe than most examples, but still an example).
The D itself didn't bother me. It's the area surrounding it that looks like glue residue or "disturbed" in some way. Good thread, though!