Deficiency of which of the given sets of elements causes necrosis?

Solution:

Necrosis, which is the death of plant tissue, particularly leaf tissue, is caused by the deficiency of certain mineral elements. The elements whose deficiency causes necrosis are Calcium (Ca), Magnesium (Mg), Copper (Cu), and Potassium (K). Other deficiencies cause different symptoms, for example, chlorosis (yellowing of leaves) is caused by the deficiency of N, K, Mg, S, Fe, Mn, Zn, and Mo. Therefore, the correct set of elements causing necrosis is Ca, Mg, Cu, and K.