The Tyrant is the final boss of the original Resident Evil.
After the player reaches the underground laboratory beneath Spencer Mansion, Umbrella’s ultimate biological weapon is released from containment. The Tyrant attacks the surviving S.T.A.R.S. members, creating the climactic boss encounter before the mansion’s destruction.
The Tyrant is a large humanoid creature with exceptional strength, durability, and a prominent claw on one arm. It represents the result of Umbrella’s T-virus weapon research and is distinct from ordinary zombies and other infected creatures encountered earlier in the game.
The game’s ending changes in details depending on the player’s route and performance, but the Tyrant remains the final major enemy encounter. Nemesis belongs to Resident Evil 3: Nemesis, William Birkin is a major antagonist in Resident Evil 2, and Jack Baker is associated with Resident Evil 7: Biohazard. Those characters are therefore plausible but incorrect answers for this question.