Trace the biochemical pathway backward from auxin to find the nutrient link.
Auxin (indole-3-acetic acid) is built from tryptophan inside the plant, so anything that blocks tryptophan formation also blocks auxin formation. Zinc is a cofactor needed for tryptophan synthase activity, which makes tryptophan from its precursors. Without enough zinc, tryptophan and hence auxin both fall, and plants show symptoms such as shortened internodes and small, rosetted leaves, classic signs of low auxin.
Calcium mainly supports cell wall and membrane function, magnesium is central to chlorophyll, and iron is needed for the electron transport chain and chlorophyll formation, none of which sits directly in the auxin synthesis pathway.
\[\boxed{\text{Zinc}}\]