Which of the following element(s) is/are confirmed by appearance of blood red colour with FeCl₃ in Lassaigne's test?

The correct option is Presence of N & S.

Let us understand the chemistry behind Lassaigne's test for the simultaneous detection of nitrogen (N) and sulfur (S) in an organic compound.

When both nitrogen and sulfur are present in an organic compound, they react with sodium metal during sodium fusion (Lassaigne's fusion) to form sodium thiocyanate (NaSCN) instead of individual sodium cyanide (NaCN) and sodium sulfide (Na₂S).
The chemical reaction during fusion is represented as follows:
$\mathrm{Na}+C+N+S\to \mathrm{NaSCN}$

When the sodium fusion extract (containing soluble NaSCN) is acidified with dilute hydrochloric acid and treated with iron(III) chloride (FeCl₃) solution, a blood-red coloration is observed.
This blood-red color is due to the formation of ferric thiocyanate complex [Fe(SCN)]²⁺ (or Fe(SCN)₃).
The reaction is represented as follows:
${\mathrm{Fe}}^{3+}+{\mathrm{SCN}}^{-}\to {\left[\mathrm{Fe}\right(\mathrm{SCN}\left)\right]}^{2+}$
(Blood-red coloration)

If only nitrogen were present, sodium cyanide (NaCN) would be formed, which gives a Prussian blue color with FeSO₄ and FeCl₃.
If only sulfur were present, sodium sulfide (Na₂S) would be formed, which gives a purple/violet color with sodium nitroprusside.
Therefore, the appearance of a blood-red color specifically confirms the simultaneous presence of both nitrogen (N) and sulfur (S) in the compound.