What is the name of the final evolution of Squirtle?

The final evolution of Squirtle is Blastoise.

Squirtle is the Water-type starter in the original Pokémon trio, alongside Bulbasaur and Charmander. In the main series games, it evolves into Wartortle and then into Blastoise, completing a three-stage evolutionary line. In the original games, Wartortle evolves into Blastoise at level 36.

Blastoise is recognizable for the two water cannons built into its shell. Its design gives the evolution line a clear progression from a small turtle to a heavily armored, artillery-like final form. Blastoise is also a pure Water type in its standard form, so it is strong against Fire-, Ground-, and Rock-type Pokémon but vulnerable to Electric- and Grass-type attacks.

The other options belong to different evolutionary lines. Charizard is Charmander's final evolution, while Wartortle is an intermediate form and Squirtle is the starting species. Later games introduced Mega Blastoise and Gigantamax Blastoise, but these are alternate powered-up forms, not separate final evolutions.