Final answer:
A branch containing three carbon atoms on the principal carbon chain is known as a propyl branch.
Step-by-step explanation:
The name of a branch on the principal carbon chain that contains three carbon atoms is a propyl branch. When naming organic molecules, the alkyl groups (branches) are named according to the number of carbon atoms they contain. For example, a one-carbon branch is called methyl, a two-carbon branch is called ethyl, and a three-carbon branch is indeed known as propyl. Similarly, a four-carbon branch would be called a butyl group.
In the context of organic chemistry nomenclature, if you have a molecule such as 3-methylpentane, the 'methyl' indicates a one-carbon branch attached to the third carbon of the principal five-carbon chain (pentane).