Which of the following alkaline earth metal does not impart characteristic colour to flame?
The correct option is C $$Be$$ Option $$(D)$$ is correct.
$$Be$$ does not impart characteristic colour to flame because its atoms are comparatively smaller and the valance electrons are strongly attached to the nucleus. Therefore its ionisation energies is high and needs large amount of energy for the excitation of their valence electrons to higher energy level which is usually not available in the Bunsen flame. So beryllium do not impart any colour to the flame.
$$Ca, Sr$$ and $$Ba$$ impart brick red, blood red and apple green colours respectively to the flame.