My cauliflower leaves are whip-like and thin and there is no curd forming, which nutrient?

Long, narrow, strap or whip-like leaves with very little leaf blade in cole crops like cauliflower and broccoli is the classic whiptail symptom of molybdenum deficiency. Molybdenum is needed for nitrogen use inside the plant, and when it is short the leaf blade fails to develop properly so you are left with mostly midrib, plus poor curd formation. Whiptail is strongly tied to acidic soils, because molybdenum becomes unavailable at low pH, unlike most micronutrients which lock up at high pH. That is an important clue: if your soil is acidic and you see whiptail, molybdenum is very likely. Please confirm with a soil test, including soil pH, before treating, since whiptail can be confused with herbicide injury or other distortions. There are two correction routes. First, because the real driver is often low pH, liming the soil to raise pH frees up the molybdenum already present and is a good long term fix. Second, a small foliar spray or seed treatment with sodium molybdate or ammonium molybdate corrects a standing or upcoming crop, and molybdenum is needed only in tiny amounts. Use very small, accurately measured doses per your soil test and local KVK advice, and do not over-apply, because molybdenum is needed in trace quantities only.