Which of the following compound will most easily be attacked by an electrophile?

Ortho, para directing groups are electron-donating groups; meta directing groups are electron-withdrawing groups. The halide ions, which are electron-withdrawing but ortho, para directing, are the exception.

Here $-\mathrm{OH}$ group is strong electron donating group. Hence electrophile can easily give electron.

$-\mathrm{O}\mathrm{H}$ group in phenol can release electrons to the ring better than $-\mathrm{C}{\mathrm{H}}_3$ group in toluene. $\mathrm{C}\mathrm{l}$ atom has electron withdrawing effect which inhibits electrophile attack readily.