Yes, you can make a custom textbox, dropdown, radio, or checkbox question required.
To add a required question, go to the “Additional Questions” section 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 choose Checkbox, click "Require the donor to check this".
To make a custom textbox required, toggle on "Require the donor to fill this textbox".
To make a custom dropdown required, toggle on "Require donor to select an option".
If you choose radio, toggle on "Require the donor to check this".
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.