Who is revealed as Eric Cartman's biological father?

Eric Cartman's biological father is Jack Tenorman.

The series first created uncertainty about Cartman's parentage by suggesting that his mother, Liane, was intersex and could also be his father. That explanation was later revealed to be a deception. In the season-fourteen episodes “200” and “201,” the truth emerges: Jack Tenorman, a former Denver Broncos player, fathered Cartman during an affair with Liane Cartman.

The revelation creates one of South Park's darkest connections between separate storylines. Jack Tenorman is also Scott Tenorman's father, making Scott Cartman's half-brother. Earlier, in “Scott Tenorman Must Die,” Cartman arranged for Scott's parents to be killed and unknowingly had Jack's remains made into chili that Scott ate. The later paternity revelation therefore makes Cartman's revenge scheme even more horrifying.

A common mistake is treating the early intersex-parent story as the final answer. It is not: the show's later continuity explicitly identifies Jack Tenorman as Cartman's biological father.