Q.Those elements impart colour to the flame on heating in it, the atoms of which require low energy for the ionisation (i.e., absorb energy in the visible region of spectrum). The elements of which of the following groups will impart colour to the flame? (Note: more than one of the given options may be correct.)

The key idea is the question's own criterion: elements whose atoms need only a low energy to excite/ionise their valence electrons emit light in the visible region when heated, and so colour the flame.

1. Group 1 (alkali metals) have the lowest ionisation enthalpies of all elements — their single valence electron is easily excited, and its de-excitation emits visible light (Li — crimson, Na — golden yellow, K — lilac). They impart colour to the flame. …

Concept and Intuition

When certain substances are heated in a flame, thermal energy excites their valence electrons to higher energy levels. The excited states are unstable, and as the electrons return to their ground state they emit the absorbed energy as photons. A characteristic colour appears only when that emitted energy falls in the visible region of the spectrum (about 400–700 nm).

The question's own criterion makes the selection explicit: the elements that colour a flame are "the atoms of which require low energy for the ionisation". Loosely-held valence electrons (low ionisation enthalpy) are also easily excited by the comparatively small energy a flame provides.

Step-by-Step Solution

1. Group 1 (alkali metals): the lowest ionisation enthalpies in the entire periodic table — a single, well-shielded $ns^1$ valence electron far from the nucleus. Flame energy excites it easily, and the de-excitation photons are visible: lithium gives crimson red, sodium golden yellow, potassium lilac. Group 1 qualifies. …