Bakelite is obtained from phenol by reacting with

Bakelite is a thermosetting polymer belonging to the family of phenol-formaldehyde resins.

Step 1: Phenol is first treated with formaldehyde ($$HCHO$$) in the presence of either an acid or a base catalyst.
  • Formaldehyde reacts at the ortho and para positions of phenol to give ortho- and para-hydroxymethyl phenols: $$C_6H_5OH + HCHO \rightarrow C_6H_4(OH)(CH_2OH)$$.

Step 2: These hydroxymethyl phenols undergo further condensation, eliminating water and building up a three-dimensional, cross-linked network. On heating under pressure, this network hardens irreversibly to give Bakelite.

Because the reaction specifically requires formaldehyde, the correct reagent among the given choices is $$HCHO$$.

Options A, B and C provide dihydric alcohol, acetaldehyde and acetone respectively; none of these carbonyl compounds can form the highly cross-linked phenol-formaldehyde network needed for Bakelite.

Hence, the correct answer is
Option D which is: $$HCHO$$