Examining the compounds from a distance suggests these conclusions, with little or no extensive knowledge of chemistry:
B is more dense than A and C is more dense than B, because you know that hydrogen is the lightest element (the least dense, because it is used in blimps) and chlorine is denser (it sinks and was used as a poison gas because it hugs the ground).
D is more dense than C, because bromine is farther down the periodic table than chlorine, and is likely to be denser.
Therefore D is the densest compound. Only one choice puts D as the densest: #1.
It seems to me that the intricacy of the question, and/or the difficulty, lies in the wording: what is decreasing order of densities?
At the very least, it should be obvious that D is the extreme compound, but a confusion between the meaning of increasing and decreasing could suggest that #4 is the correct choice. A little reflection rules that out: the density of D cannot be less than that of A!
The density of 1-bromo 3-chlorobenzene was not available in my search, but for the others, benzene has 0.876, chlorobenzene 1.11, m-dichlorobenzene 1.228, and bromobenzene 1.5. Bromochlorobenzene would probably come in at about 1.75. The actual densities are not at all similar, so the question must have been going after something else.
The effect of packing efficiency, due to dipole moment - in liquids - is probably much, much smaller. One challenging possibility would be that one of the compounds, perhaps C or D, could be a solid - but, no, they are all liquids.
Maybe it was a freebie: you have to work at it to get it wrong.