Which conformation of ${C_6H_6Cl_6}$ is the most powerful insecticide?

Which of the following angles in ethane remains constant,and which angle changes during rotation about the $C-C$ single bond?

Which is the most stable conformer along the $2, 3$-$C-C$ bond axis of the compound?

$HO-CH_2-CH_2-CHO$
Which conformer of the above compound is most stable (consider the conformer across the $C_2-C_3$ bond)?

One of the configurations of $n$-butane is shown in the given figure. An anticlockwise rotation of $C_2$ around the $C_2-C_3$ bond by $120^\circ$ will lead to:

In $ethane$ and $cyclohexane$,which one of the following pairs of conformations are more stable?