1. Identify the reaction type

Acetylene is ethyne, $HC\equiv CH$.

Hypochlorous acid is $HOCl$, which adds across a multiple bond. For alkynes, addition of $HOCl$ gives a vinyl halohydrin, which then undergoes keto-enol tautomerism to form a carbonyl compound.

2. Addition of $HOCl$ to acetylene

Across the triple bond of acetylene:

$HC\equiv CH + HOCl \rightarrow CHCl=CHOH$

This is the enol form.

3. Tautomerism of the enol

The enol $CHCl=CHOH$ tautomerizes to the corresponding aldehyde:

$CHCl=CHOH \rightleftharpoons ClCH_2CHO$

So the product is chloroacetaldehyde.

4. Match with options

• A: $CH_3COCl$ = acetyl chloride, not formed here
• B: $ClCH_2CHO$ = chloroacetaldehyde, correct
• C: $Cl_2CHCHO$ = dichloroacetaldehyde, would require two Cl atoms
• D: $ClCHCOOH$ = not the product of this addition

5. Conclusion

The product formed when acetylene reacts with hypochlorous acid is:

$\boxed{ClCH_2CHO}$

So the correct option is B.