Which of the following structure of butane?

Compounds made of carbon atoms can have a variety of configurations, including long chains, branching chains, and cyclic chain structures. 

A hydrocarbon with four carbon atoms has a chemical formula  ${\mathrm{C}}_4{\mathrm{H}}_6$.
The following two types of structural formulas are available for butane. This implies that there are two possible arrangements for four carbon atoms. The carbon atoms in isobutane are organized in a branching chain.
The carbon atoms in n-butane are organized in a linear chain.

 isobutane

    n-butane