Explanation

(d)
In electrothermal process silica is heated with calcium phosphate when phosphorus pentoxide is obtained .It is then reduced by coke in electric furnace to get white phosphorus.
2Ca_{3}(PO_{4})_{2}+6SiO_{2}$\overset{\mathrm{\Delta}}{\rightarrow}$6CaSiO_{3}+P_{4}O_{10}
P_{4}O_{10}+10C$\overset{\mathrm{\Delta}}{\rightarrow}$ P_{4} + 10CO