n-pentane and isopentane are _____________ isomers.

Chain
Position
Geometrical
Optical

n-Pentane and isopentane are **chain isomers**. Here's why:

- **Chain Isomers**: These are compounds with the same molecular formula but different carbon chain structures. n-Pentane (also known as normal pentane) and isopentane (also known as methylbutane) both have the molecular formula \( C_5H_{12} \), but their structures differ in the arrangement of the carbon chain.

- **n-Pentane** has a straight chain of five carbon atoms.
- **Isopentane** has a branched chain with four carbon atoms in the main chain and one branch with a single carbon atom.

- **Position Isomers**: These are isomers that differ in the position of a substituent group on the carbon chain. This is not the case for n-pentane and isopentane.

- **Geometrical Isomers**: These occur due to the presence of a double bond or ring structure, which allows for cis/trans or E/Z isomerism. This is not relevant to n-pentane and isopentane as they are alkanes with single bonds only.

- **Optical Isomers**: These are isomers that have chiral centers and exhibit optical activity. Since neither n-pentane nor isopentane has a chiral center, they do not exhibit optical isomerism.

So, the correct answer is:

\[1) \text{Chain} \]