In the following sequence of reactions the correct, P, Q and R are respectively

​$$\text{P (in dil. AcOH)} \xrightarrow{\text{thiourea}} \text{solid} + \text{N}_2 \text{ gas} + \text{Solution} \xrightarrow{\text{dil. FeCl}_3} \text{Q (blood red soln.)} \xrightarrow{\text{dil. HgCl}_2} \text{R (colourless)}$$​

Correct option is B

​Potassium nitrite (distinct from potassium nitrate) is an inorganic compound with the chemical formula KNO₂. It is an ionic salt of potassium ions K⁺ and nitrite ions NO₂^-, which forms a white or slightly yellow, hygroscopic crystalline powder that is soluble in water. Thiourea is also known as thiocarbamide. Thiourea, also known as thiocarbamide, is an organic molecule that is similar to urea but includes sulfur rather than oxygen; its chemical formula is CS(NH₂)₂, whereas ureas are CO(NH₂)₂.

In dilute acetic acid, potassium nitrite reacts with thiourea and forms thiocyanate ion (SCN^-) with the formation of N₂ gas

$$KNO_2 + NH_2CSNH_2 + \text{AcOH (dilute)} \rightarrow N_2 + \text{SCN}^- + H_2O$$

The thiocyanate ion reacts with Fe(III) ion which gives blood red colour due to the formation of Fe(SCN)₃.

$$\text{FeCl}_3 + 3\text{NH}_4\text{SCN} \rightarrow \text{Fe(SCN)}_3 + 3\text{NH}_4\text{Cl}$$

The addition of HgCl₂ to the reaction mixture decreases the intensity of the red color and the solution becomes colorless because of the formation of a stable complex.

$$\text{HgCl}_2 + \text{Fe(SCN)}_3 \rightarrow [\text{Hg(SCN)}_4]^{2-}$$