Which of the following is trihydric phenol?

Phenols are classified by the number of hydroxyl groups bonded to the benzene ring: monohydric (one $\mathrm{-OH}$), dihydric (two $\mathrm{-OH}$), or trihydric (three $\mathrm{-OH}$).

Catechol (benzene-1,2-diol), resorcinol (benzene-1,3-diol), and quinol (benzene-1,4-diol) are all dihydric phenols.

Pyrogallol is benzene-1,2,3-triol and contains three hydroxyl groups.

Thus, pyrogallol is the only trihydric phenol among the options.