taxiing for departure from runway 21 - along taxiway C - I initially thought the sign D was referring to the taxiway north of the intersection with 12/30 - but saw more C signs - so reached the conclusion that the taxiway south of 12/30 is D.
I'll have another chance to look at the signs.
The US AIP does not contain an aerodrome chart for KAOO unfortunately - and I haven't found any free aerodrome charts on the web.