Explanation

(d)
Flame colours are due to metal excess defect. What happens that in some ionic crystals, there becomes an excess of metal atom, which by loosing $e^{-},$ change into ions. These electrons can absorb energy and go into excited states from ground state. Thus, the absorption of certain wavelength of light takes place and crystal becomes coloured according to complementary colour. The spaces occupied by extra $e^{-}$ are called F-centres.