The question involves identifying two chemical species, P and Q, based on a series of reactions.

Identifying Red Vapour P

When a mixture of sodium chloride (NaCl), concentrated sulfuric acid ($H_2SO_4$), and potassium dichromate ($K_2Cr_2O_7$) is heated, a characteristic red vapour is formed. This vapour is chromyl chloride ($CrO_2Cl_2$).

The reaction is:

$ K_2Cr_2O_7 + 4NaCl + 6H_2SO_4 \xrightarrow{\Delta} 2CrO_2Cl_2 \uparrow + 2K_2SO_4 + 6H_2O $

Therefore, the red vapour (P) is $CrO_2Cl_2$.

Formation of Yellow Solution and Red Solid Q

The red vapour, chromyl chloride ($CrO_2Cl_2$), dissolves in aqueous sodium hydroxide (NaOH). This reaction leads to the formation of sodium chromate ($Na_2CrO_4$), which gives the solution a yellow colour.

The simplified reaction is:

$ CrO_2Cl_2 + 2NaOH \rightarrow Na_2CrO_4 + 2HCl $

The yellow solution contains sodium chromate ($Na_2CrO_4$).

When this yellow solution is treated with silver nitrate ($AgNO_3$), a red precipitate of silver chromate ($Ag_2CrO_4$) is formed.

The reaction is:

$ Na_2CrO_4 + 2AgNO_3 \rightarrow Ag_2CrO_4 \downarrow + 2NaNO_3 $

The red solid (Q) is $Ag_2CrO_4$.

Conclusion

Based on the reactions:

• P = $CrO_2Cl_2$ (chromyl chloride)
• Q = $Ag_2CrO_4$ (silver chromate)

The correct option is the one that lists $CrO_2Cl_2$ and $Ag_2CrO_4$ respectively.