Fields are how you could do this with older doc files. If you are using docx files content controls are a better solution because the result is updated instantly (don't need fields to update to see the result). You also don't need to add multiple or nested fields to cater for each possible choice.

An example with linked content controls is demonstrated in https://www.msofficeforums.com/130893-post3.html

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Andrew Lockton
Chrysalis Design, Melbourne Australia