Yes, you can make a custom textbox, dropdown, or checkbox question required. To add a required question, go to the “Advanced Setup” tab of your donation form editor and enable "Ask additional questions." Once you've added your question, select either Checkbox, Textbox, or Dropdown as your answer value input type. If you have chosen "Checkbox" click "Require the donor to check this."
To make a custom textbox required, check the "Require the donor to fill this textbox" option.
To make a custom dropdown required, check the "Require donor to select from the dropdown without using the first value" option.
As you can see below, if your donors forget to click the required checkbox, they will be notified and unable to move along the donation process until they have done so.