Is often a major part of fertilizers since the plant cannot usually produce (or obtain) sufficient amounts. which of these is not a recognized class of plant hormone?
food
sugars
nitrogen
potassium

The answer you are looking for is C. Nitrogen. Commonly found as Nitrite 
(N3) or Nitrate (N2) nitrogen is a key element required by all life. Nitrogen is most often released back into the atmosphere by micro-organisms during the decomposition of organic matter, and into the ground through decomposition and defecation (Pooping).