The answer depends on your parents' marital status and who you live with the most.
Mos's financial aid tool will automatically walk you through this step by step and specify when a question applies to a specific parent.
Here's how it's worked out:
If your parents are married, or were never married and are still living together, then you will provide information (such as name, birthday, and financial information) for both parents.
If your parents are divorced or separated and living apart, then you should provide answers for the parent with whom you lived the most in the last year. If you lived with both parents equally, then you will be asked to supply information for the parent who provided you with more financial support. For financial aid purposes, this parent is often referred to as your "custodial parent."
If your custodial parent is remarried, then you must also include information for your step-parent.
The U.S. Department of Education publishes an infographic to help illustrate this here: