Finding reactants in a azo-coupling reaction whose product is given
The comments summarized the reasons well: the diazotization reaction for the initial structure would produce a mixture of $\alpha$-azo and $\beta$-azo compounds, which would give a mixture of products. Also, diazonium salts couple with activated aromatic systems. The diazonium salt shown would not couple with nitrobenzene, which is highly deactivating and does not favour electrophilic aromatic substitution.
Also, having an electron withdrawing group on the diazonium salt favours the reaction. Thus, the correct solution would have the azo group on the smaller, less activated benzene ring rather than the larger, more activated naphthalene ring.