Answer: The covenant being made with Adam as a public person, not for himself only, but for his posterity, all mankind descending from him by ordinary generation1, sinned in him, and fell with him in that first transgression₂.

1. Acts 17:26
2. Genesis 2:16-17; Romans 5:12-20; 1 Corinthians 15:21-22

For further study see:

1. WSC 16: Did all mankind fall in Adam’s first transgression?
2. Elder Gregory Moeck of First Presbyterian Church in San Francisco on WLC 22a: “Guilt of Adam’s transgression”
   
3. Elder Gregory Moeck of First Presbyterian Church in San Francisco on WLC 22b: “How can I be guilty of Adam’s sin?”