What selection option allows users to choose only one item from a predefined set?

The choice that allows users to select only one item from a predefined set is the radio button. This interface element is specifically designed for situations where the user is presented with multiple options, but can only select one at a time. When a radio button is used, selecting a new option automatically deselects the previously chosen option, which ensures that only a single choice is active at any given time.

Checkboxes, on the other hand, allow multiple selections, as each checkbox operates independently from the others. A dropdown list, while it can limit the selection to one item at a time as well, can sometimes allow for different interactions, such as multiple selections in certain contexts, though typically it is designed for single selection. Text input fields do not provide predefined options; instead, they allow users to enter data manually and can lead to varied responses not constrained to a set list. Therefore, radio buttons are the optimal choice for enforcing a single selection.