A is very correct, magnesium is the correct answer, did you know the meaning of the term "chlorosis"?, the term "chlorosis means yellow colouration, sometimes you will observed that yellow leaves appear on a tree or plants, this colouration did not just formed it's formed due to lack of some minerals or nutrients, the minerals that plants needs is broadly divided into two...micro nutrient(small), and major nutrient (large) they are required in large and small quantities...the major nutrients include chlorine(C), nitrogen (N), phosphorus(P), pottasium(K), sodium(NA),sulphur (S), calcium(CA), magnesium(M)....CNPKNASCAM, while the one required in small quantities include maganese, zinc, flourine, iron and so on.if one of them is lacked in a plants it's going to show some signs which that of magnesium leads to chlorosis (yellow colouration).