# The $C-H$ bond distance is the longest in

$(a)\;C_2H_2 \\ (b)\; C_2 H_4 \\ (c) C_2 H_6 \\ (d) C_2 H_2 Br_2$

$C_2H_6$ is an alkane $(sp^3 )$ hybridisation.
Among alkanes, alkenes and alkynes and so the $C-H$ bond length will be maximum in case of alkanes.
Hence c is the correct answer.