It means $-CHO$ group is present.
It means only one $-CHO$ group is present.
${C_2}{H_5}OH + C{l_2} \to C{H_3}CHO + 2HCl$
$C{H_3}CHO + 3C{l_2} \to \mathop {CC{l_3}CHO}\limits_{{\rm{Chloral}}} + 3HCl$
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$\underset{(A)}{\mathop{\begin{matrix}
O \\
|| \\
C{{H}_{3}}-C-C{{H}_{3}}, \\
\end{matrix}}}\,\,\,\,\,\underset{(B)}{\mathop{\begin{matrix}
\,\,\,\,\,\,O \\
\,\,\,\,\,\,|| \\
C{{H}_{3}}-C-H, \\
\end{matrix}}}\,\,\,\,\,\underset{(C)}{\mathop{\begin{matrix}
O\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
||\,\,\,\,\,\,\,\,\,\,\, \,\, \\
C{{H}_{3}}-C-C{{H}_{2}}-Cl, \\
\end{matrix}}}\,$ $\,\,\underset{(D)}{\mathop{\begin{matrix}
O \\
|| \\
H-C-H, \\
\end{matrix}}}\,\,\,\,\underset{(E)}{\mathop{\begin{matrix}
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O \\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|| \\
Cl-C{{H}_{2}}-C-H \\
\end{matrix}}}\,$
$A\xrightarrow{NaN{{O}_{2}}/HCl}B\xrightarrow{KCN}C\xrightarrow{{{H}_{3}}{{O}^{+}}}$ benzoic acid
The compound $ A$ is