Final answer:
Acetaldehyde (D) is the most reactive compound towards a nucleophilic addition reaction because it has an electrophilic center.
Step-by-step explanation:
In nucleophilic addition reactions, a nucleophile attacks an electrophile and forms a new bond. The compound that is most reactive towards a nucleophilic addition reaction is one that has an electrophilic center which is easily attacked by a nucleophile. Among the given options, the compound that has an electrophilic center is acetaldehyde (D). Acetaldehyde has a carbonyl group (C=O) which is an electrophilic center and can easily undergo nucleophilic addition reactions.