Which studio developed Halo: Reach?

Bungie developed Halo: Reach.

Halo: Reach was the final Halo game developed by Bungie before the studio ended its direct development relationship with the franchise. Microsoft Game Studios published the game for Xbox 360 in 2010. The story is a prequel to Halo: Combat Evolved and follows Noble Team during the Covenant invasion of the human planet Reach.

The player controls Noble Six, a new member of Noble Team. Although the game takes place before Master Chief’s first campaign, it connects directly to the original game through the fall of Reach and the escape of the starship Pillar of Autumn.

Ensemble Studios developed Halo Wars, while 343 Industries later became the primary developer of mainline Halo titles. Creative Assembly developed Halo Wars 2. Those studios are therefore plausible alternatives in a Halo development question, but Halo: Reach itself was made by Bungie.