How can I add a ticket field where a customer can only select one of multiple options?

A solution for this would be to create a custom drop-down field. This is an icon with an arrow pointing downwards found in the "Drag and Drop field."

Please navigate to Admin > Workflows > Ticket Fields. Drag and drop the 'Drop-down' field, enter the multiple choices and choose the desired settings where you could display it to the customer and give them access to edit. 

Kindly click on Done and Save on the Tickets fields page for this to be available in the ticket form.