Which Study Would Be Considered Landmark Research

Which study would be considered landmark research?

A) The discovery that during pancreatitis, the pancreas is actually digested by its own enzymes
B) A description of the importance of injecting immunizations in the proper location of the deltoid
C) The third in a series of four papers describing patterns of emergency room use in vacation communities
D) The first paper describing the effect of using insulin for type I diabetes in humans