To determine which compound is most reactive towards nucleophilic addition reactions, we need to consider the electronic effects of substituents on the carbonyl group.$\newline$Nucleophilic addition reactions typically occur at the carbonyl carbon, which is electrophilic. The reactivity depends on the electron-withdrawing or electron-donating nature of substituents.$\newline$Let's analyze each option:$\newline$Option 1: Benzaldehyde (C$_6$H$_5$CHO)$\newline$• The phenyl group is slightly electron-withdrawing due to resonance, but it also donates electrons through hyperconjugation.$\newline$Option 2: Acetophenone (C$_6$H$_5$COCH$_3$)$\newline$• The methyl group is electron-donating, reducing the electrophilicity of the carbonyl carbon.$\newline$Option 3: p-Tolualdehyde (p-CH$_3$C$_6$H$_4$CHO)$\newline$• The methyl group is electron-donating, which decreases the reactivity of the carbonyl group.$\newline$Option 4: p-Nitrobenzaldehyde (p-NO$_2$C$_6$H$_4$CHO)$\newline$• The nitro group is a strong electron-withdrawing group, increasing the electrophilicity of the carbonyl carbon.$\newline$Conclusion:$\newline$• The presence of the nitro group in p-nitrobenzaldehyde makes the carbonyl carbon more electrophilic and reactive towards nucleophilic addition.$\newline$Therefore, the most reactive compound towards nucleophilic addition is Option 4: p-Nitrobenzaldehyde.