1. Go to Design or Layout view of the form
2. Right click the field, select "Properties"
3. Go to "Data" tab
4. Change "Row Source" Type to "Value List"
5. Enter list of choices under "Row Source", e.g. "Yes";"No", or click "..." to enter
You may want to add below for data validation to prevent accidentally direct data inputs to the linked table (not necessary)
1. Go to the Design view of the table linked to the form.
2. Select Field ==> go to "General" tab at the bottom.
3. Go to "Validation Rule" and enter list of choices, e.g. "Yes";"No"