What does the prefix "iso-" typically refer to in organic chemistry?

In organic chemistry, the prefix "iso-" specifically refers to a branched structure, commonly indicating that the compound has a specific arrangement where the longest continuous carbon chain has a branch at the second carbon. This term is often used with alkanes, such as isobutane, where the structure features a three-carbon chain with a methyl group as a branch on the second carbon. This branching is a key characteristic that distinguishes these compounds from their straight-chain counterparts.

The other choices do not accurately reflect the meaning of “iso-.” For instance, the prefix does not pertain to cyclic structures, nor does it imply the presence of two identical groups on the same carbon or that all carbon atoms must be in a straight chain. Thus, the use of "iso-" is specifically tied to the notion of branched hydrocarbons, supporting why it accurately corresponds to the correct answer choice regarding organic compound nomenclature.