The chemical composition of Tollens' regent is

Ammoniacal silver nitrate solution

The correct answer is Option (4) → Ammoniacal silver nitrate solution

Tollens' reagent (used to test for aldehydes, giving the silver mirror test) is prepared by adding aqueous ammonia (NH₃) to silver nitrate (AgNO₃) solution until the initial precipitate of Ag₂O redissolves. This forms the diamminesilver(I) complex [Ag(NH₃)₂]⁺ in solution.

• Chemical composition: Ammoniacal solution of silver nitrate (or more precisely, [Ag(NH₃)₂]NO₃ in aqueous NaOH/ammonia).

The other options are unrelated:

• Sodium bicarbonate: Buffer, not a reagent for aldehyde test.
• Mercurous chloride: Calomel, used in reference electrodes.
• Iodoform: CHI₃, product of iodoform test for methyl ketones/acetaldehyde.

Thus, the correct answer is Ammoniacal silver nitrate solution.