Technically speaking, a D grade means a person is up to mark(i.e. as in perform their duties adequately). C and above actually means a person performs better than adequate in their duties at a few areas at least. Hence going by HR guidelines, if you just perform your normal duties, they will take it as 'fair' to give you a D.
However most of the time Ds are not doled out much as the quota for C grades are pretty big. Bell curve or not, if you get a D you have to ask yourself did you screw up somewhere to be graded like that.