Thermosetting polymer,

Bakelite is a thermosetting polymer formed by the condensation reaction of phenol with $\mathrm{HCHO}$ in the presense of conc. $\mathrm{H}_{2} \mathrm{SO}_{4}$

It is thus, a cross-linked polymer in which condesation takes place at O- and p- positions So HCHO is the corerect option.