The final product formed when acetylene reacts with hydrogen bromide is

$\mathrm{HC}\equiv \mathrm{CH}+\mathrm{HBr}\to {\mathrm{CH}}_2=\mathrm{CHBr}$

Acetylene on reaction with hydrogen bromide forms vicinal bromide which on further reaction with hydrogen bromide gives the final product as ethylidene bromide.

${\mathrm{CH}}_2=\mathrm{CHBr}+\mathrm{HBr}\to {\mathrm{CH}}_3-{\mathrm{CHBr}}_2$

Therefore, option (C) is correct.