Recall the classic disorder linked with each named nutrient in cauliflower.
Zinc shortage typically causes little leaf and interveinal chlorosis, iron shortage causes interveinal chlorosis in young leaves, and nitrogen shortage causes overall pale, stunted growth. None of these matches the whiptail symptom described for cauliflower.
Whiptail, where cauliflower leaves narrow down to just the midrib without a proper blade, is the textbook diagnostic sign of molybdenum deficiency, which is why cauliflower is the standard indicator crop for this deficiency.
\[\boxed{\text{Molybdenum}}\]