Explanation:

Whiptail disease in cauliflower is a classic symptom of Molybdenum (C) deficiency. Molybdenum is crucial for nitrate reductase and nitrogenase enzymes; its deficiency leads to distorted, strap-like leaves. Magnesium (A) deficiency causes interveinal chlorosis. Boron (B) deficiency affects growing points and fruit set. Zinc (D) deficiency typically causes 'little leaf' or 'rosette' symptoms. Thus, (C) is the specific cause.