I don't know for sure, but I'm fairly certain "C" means a string bend and "D" means a release. Here's why I think so:
Notice the tab in the rhythm part. Where the "C" occurs, you play the D-string, 4th fret---that's an F#. And sure enough, the standard notation shows an F#. But then the standard notation shows that note becoming a G, while the tab continues to show D-string, 4th fret. The only way to play a G at that string/fret is to bend the string a half-step.
And then the standard notation shows the note going back to an F# at the same place where the "D" occurs, which makes me think the "D" indicates releasing the bend.
If you look at the solo, you'll see the same thing occur. At the beginning of the solo, the tab shows B-string, 12th fret -- that's a B. Then it's marked with that "C" marking. The standard notation shows a B grace note going up to a C, which again could only be achieved on the 12th fret of the B-string by bending it a half-step. And then the pitch of the note goes from that high C back down to the B, while the tab stays on the 12th fret, but this time marks it with "D".
Basically, wherever C is notated, the pitch of the note goes up, but the fret stays the same, and when D is notated, the pitch goes back down again. Notice, too, that "C" and "D" seem to come in pairs.
So again, I don't know for certain, but this is my educated guess: "C" means "bend the string" and "D" means "release the bend".