Per the rules,
DC counts as Maryland,
so the Identifier should be "MDC"
I read the NAQP rules carefully and I don't think that's right.
In the sweepstakes or field day, MDC (section designation) is clearly the right thing. But NAQP is not SS or field day. NAQP uses the state.
The only words in the NAQP that address DC are that "District of Columbia counts as Maryland". I would take that to mean that DC stations can identify themselves as being in DC and the other station is supposed to know that it is the same mult as Maryland. Others might take this as meaning that DC stations are supposed to send MD but I don't think this is preferable.
Again, don't confuse ARRL Section with state. I think the only answer so far that is clearly not right, is MDC. Somebody in California wouldn't send LAX or EB in the NAQP.