The more nucleophilic group will make ring more prone to aromatic substitution reaction. Benzene becomes more reactive towards Electrophilic Aromatic Substitution when any substituent raises its electron density.

In structure (a) $-{\mathrm{CH}}_2$ group shows $+\mathrm{H}$ effect. while in (b) $-\mathrm{OH}$ group shows $+\mathrm{R}$ effect. In (c) $+\mathrm{R}$ effect is shown by functional group and in $\left(\mathrm{d}\right)$ both $-\mathrm{OH} \mathrm{and} -\mathrm{O}-$ show $+\mathrm{R}$ effect. 

So Correct increasing order is :

$\mathrm{c} < \mathrm{a} < \mathrm{b} < \mathrm{d}$