Isopentane, also called ________, is a branched-chain saturated hydrocarbon with the chemical formula C₅H₁₂.

2-methylbutane

The correct answer is option 3. 2-methylbutane.

Isopentane, also known as 2-methylbutane, is a type of hydrocarbon. Let's break down the name and chemical structure:

1. Isopentane: The prefix "iso-" indicates that the molecule is an isomer of pentane, which is a straight-chain hydrocarbon with five carbon atoms \((C_5H_{12})\). Isopentane is an isomer because it has the same molecular formula \((C_5H_{12})\) as pentane, but its atoms are arranged differently.

2. 2-methylbutane: This name provides additional information about the structure of the molecule. "2-methyl" indicates that there is a methyl group \((-CH_3)\) attached to the second carbon atom of the parent chain. "Butane" suggests that the parent chain consists of four carbon atoms (but-) and is saturated (-ane), meaning it contains only single bonds between carbon atoms.

3. Chemical Formula: The chemical formula of isopentane (2-methylbutane) is \(C_5H_{12}\). This formula indicates that the molecule contains five carbon atoms \((C_5)\) and twelve hydrogen atoms \((H_{12})\).

4. Structure: The structural formula of isopentane (2-methylbutane) can be represented as follows:
 
 
In this structure, the central carbon atom is bonded to three hydrogen atoms and one methyl group, and each of the outer carbon atoms is bonded to two hydrogen atoms.

Overall, isopentane (2-methylbutane) is a branched-chain saturated hydrocarbon with five carbon atoms and twelve hydrogen atoms. It is commonly used as a fuel and as a refrigerant.