The correct decreasing order of densities of the following compounds is:

Density of aromatic halogen compounds increases with:

1. Increase in number of halogen atoms

2. Increase in atomic mass of halogen atoms

This is due to increase in molar mass.

(A)

$$\mathrm{C_6H_6}$$

(Benzene)

Contains no halogen atom.

Hence, it has the lowest density.

(B)

$$\mathrm{C_6H_5Cl}$$

(Chlorobenzene)

One hydrogen atom is replaced by chlorine.

Since chlorine is heavier than hydrogen, density increases.

(C)

$$\mathrm{C_6H_4Cl_2}$$

(1,3-Dichlorobenzene)

Contains two chlorine atoms.

Hence, density is greater than chlorobenzene.

(D)

$$\mathrm{C_6H_4BrCl}$$

(1-Bromo-3-chlorobenzene)

Contains one bromine and one chlorine atom.

Since bromine has much larger atomic mass than chlorine:

$$\mathrm{Br > Cl}$$

this compound has the highest density.

Therefore, decreasing order of density is:

$$\mathrm{(D) > (C) > (B) > (A)}$$