Among the following the order of reactivity towards nucleophilic addition is

The nucleophilic addition to a carbonyl group occurs when the nucleophile attacks the electrophilic carbon of the $$\mathrm{C}= \mathrm{O}$$ bond.
The rate of this attack depends on two key factors:

(i) Electronic effect (−I / +I): Any group that donates electron density by the +I effect decreases the partial positive charge on the carbonyl carbon, making it less electrophilic.
(ii) Steric effect: Bulky groups around the carbonyl carbon hinder the approach of the nucleophile.

Analyse each compound:

1. Formaldehyde, $$\mathrm{HCHO}$$
• Substituents: two hydrogens.
• Hydrogens exert almost no +I effect and offer practically no steric hindrance.
⇒ The carbonyl carbon is highly δ⁺ and fully exposed, so reactivity is maximum.

2. Acetaldehyde, $$\mathrm{CH_3CHO}$$
• Substituents: one methyl, one hydrogen.
• The methyl donates electron density (+I) and adds some bulk, but only on one side.
⇒ Electrophilicity and accessibility are reduced compared with formaldehyde but still greater than a ketone with two methyls.

3. Acetone, $$\mathrm{CH_3COCH_3}$$
• Substituents: two methyl groups.
• Both methyls donate electron density (+I) and create significant steric crowding.
⇒ The carbonyl carbon is least δ⁺ and most shielded, so reactivity is minimum.

Combining the electronic and steric considerations:

$$\mathrm{HCHO} \; \gt \; \mathrm{CH_3CHO} \; \gt \; \mathrm{CH_3COCH_3}$$

Hence the correct order of reactivity toward nucleophilic addition is given by Option B which is: $$\mathrm{HCHO} \; \gt \; \mathrm{CH_3CHO} \; \gt \; \mathrm{CH_3COCH_3}$$.