${\mathrm{CaOCl}}_2+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}(\mathrm{OH}{)}_2+\mathrm{X}$

$\underline{X}+{\mathrm{CH}}_3\mathrm{CHO}\to \mathrm{Y}$

$\underline{\mathrm{Y}}+\mathrm{Ca}(\mathrm{OH}{)}_2\to {\mathrm{CHCl}}_3$

What is 'Y' in the above chemical reaction?

Given data : It is given to identify the Y.

Explanation
$CaOCl+H_{2}O\to Ca{\left(OH\right)}_2+ \underset{X}{C{l}_2}$
Ethanol is converted to aldehyde, which when combined with $C{l}_2$, produces $CC{l}_{3}CHO$.

$$\begin{gathered} {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}+\left[\mathrm{O}\right]\to {\mathrm{CH}}_3\mathrm{CHO} \\ {\mathrm{CH}}_3\mathrm{CHO}+3{\mathrm{Cl}}_2\to \underset{Y}{{\mathrm{CCl}}_3\mathrm{CHO}}+3\mathrm{HCl} \end{gathered}$$$\mathrm{Ca}(\mathrm{OH}{)}_2+{\mathrm{CCl}}_3\mathrm{CHO}\to \mathrm{Ca}{\left(\mathrm{HCOO}\right)}_2+{\mathrm{CHCl}}_3$

from the above reaction

$\mathrm{X}={\mathrm{Cl}}_2\text{ and }\mathrm{Y}={\mathrm{CCl}}_3\mathrm{CHO}$

Hence, the correct option  (C). ${\mathrm{CCl}}_3\mathrm{CHO}$.