The colour of the flame that is observed when a substance is burnt or ignited in air, is a characteristic property and unique to each element or compound. It can also be used as a measure of detection or identification of the various elements or compounds associated with it.
Complete step by step answer:
When we give energy to any substance, the kinetic energy of the particles of the substance increases. This means that the elementary particles absorb this energy and are at elevated positions on the energy graph. If the energy transmitted to particles crosses a certain value, that substance then catches fire and bursts into flames.
The colour of light emitted by these flames depends on the energy that is emitted by each electron while it is returning to its original energy state. Since the electronic configuration of different elements is different, the number of electrons in the valence shell also differ. This means that there might be more or less electrons elevating to a higher energy state and then returning back to give the flame its colour. Hence, different substances may have different colours of flame.
In accordance with the theory presented above, the colours of the flames of the given elements, namely Li, Na, K and Rb and their salts are crimson red, golden yellow, lilac and red-violet respectively.
Hence, Option D is the correct option.
Since the electronic configuration of the metals can be exhibited in the salts that they form with other anions, their corresponding salts too carry on the same colour for the flame most of the time.