The correct answer is B. chain isomer.
Chain isomers are compounds that have the same molecular formula but different structural formulas. They differ in the order in which the carbon atoms are bonded together.
Normal butane and isobutane are both isomers of butane, with the molecular formula $C_4H_{10}$. However, they have different structural formulas:
- Normal butane has a straight chain of carbon atoms:
$$CH_3-CH_2-CH_2-CH_3$$
- Isobutane has a branched chain of carbon atoms:
$$CH_3-CH(CH_3)-CH_3$$
The two compounds have different physical properties, such as boiling point and melting point. Normal butane has a boiling point of $-0.5^\circ$ C and a melting point of $-138^\circ$ C, while isobutane has a boiling point of $-11.7^\circ$ C and a melting point of $-159^\circ$ C.
The two compounds also have different chemical properties. Normal butane is more reactive than isobutane, because the branched chain of carbon atoms in isobutane makes it more difficult for the molecule to react.