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Chlorobenzene needs very harsh conditions for substitution of Cl by OH, whereas in 2,4-dinitrochlorobenzene the chlorine is displaced much more easily. The reason is that

1. the NO2 groups increase electron density at the ortho and para positions of the ring
2. the NO2 groups pull electron density away from the meta position
3. the NO2 groups donate electron density at the meta position
4. the NO2 groups withdraw electron density from the ortho and para positions

Correct answer: the NO2 groups withdraw electron density from the ortho and para positions

Solution

The -NO2 groups at the 2- and 4-positions withdraw electron density (by -I and -M effects) from the ortho and para carbons, stabilising the Meisenheimer intermediate and so making nucleophilic displacement of Cl by OH much easier.

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