Bungie developed Halo: Combat Evolved before the franchise moved to 343 Industries.
Halo: Combat Evolved was created by Bungie and published by Microsoft Game Studios for the original Xbox in 2001. Bungie’s work established the series’ core combination of first-person shooting, vehicle combat, science-fiction world-building, and orchestral music. The game also helped define the original Xbox as a major console platform.
Bungie continued developing the mainline series through Halo 3, Halo 3: ODST, and Halo: Reach. After Bungie separated from Microsoft, 343 Industries was established within Microsoft to manage the Halo franchise. 343 Industries developed Halo 4, Halo 5: Guardians, and Halo Infinite.
Ensemble Studios did develop Halo Wars, a real-time strategy spin-off, but it did not develop Combat Evolved. Rare is another Microsoft-associated studio, but it is not the developer of the first Halo game. The answer is therefore Bungie, the studio responsible for the original title.