An instrument that requires clients to rate their level of anxiety from (1) not at all anxious to (5) extremely anxious is a:

The correct answer is a rating scale because it is specifically designed to allow clients to express their level of anxiety quantitatively by choosing a numerical value that corresponds to their feelings. The scale's design, which ranges from 1 (not at all anxious) to 5 (extremely anxious), provides a clear and structured way to gauge the intensity of a client's anxiety.

A rating scale is effective in measuring subjective experiences, such as anxiety, by transforming qualitative feelings into quantitative data. This enables practitioners to evaluate and analyze the clients' responses more easily and systematically.

Other options, such as a survey form and a questionnaire, might encompass a broader collection of data and could include various types of questions, not limited to a numerical rating of anxiety. While a checklist generally lists items for users to mark as applicable or complete, it does not inherently measure degrees of feelings or states like anxiety, which is specific to the nature of a rating scale. Therefore, the unique function and structure of a rating scale make it the most appropriate choice for this scenario.