$(1)$ $\begin{matrix}
O \\
|| \\
{{C}_{6}}{{H}_{5}}-C-{{C}_{6}}{{H}_{5}} \\
\end{matrix}$
$(2)$ $C _{6} H _{5}- CHO$
$(3)$ $p - CH _{3}- C _{6} H _{4}- CHO$
$(4)$ $p - CH _{3} O - C _{6} H _{4}- CHO$
Therefore, the correct order is as follows :
\(2 > 3 > 4 > 1\)


$\mathop {(A)}\limits_{ gives\,\,positive ~\\ iodoform\,Test } \xrightarrow[\begin{subarray}{l} (ii)\,{H^ + },{H_2}O + ~\\ (iii)\,conc.\,{H_2}S{O_4}/\Delta \end{subarray} ]{{{\text{(i) C}}{{\text{H}}_{\text{3}}}{\text{MgBr}}}}(B)\xrightarrow{{{O_3}/Zn,{H_2}O}}$