Portal is set in the same fictional universe as which Valve series?

Portal is set in the same fictional universe as Half-Life.

Valve designed Portal as a story connected loosely to its established Half-Life setting. Both series feature the rivalry between Aperture Science and Black Mesa, two research organizations pursuing dangerous scientific breakthroughs. Portal focuses on Aperture’s testing facility, while Half-Life follows Gordon Freeman and the disasters surrounding Black Mesa.

The connection is deliberately subtle rather than a direct crossover. Portal takes place around the period between Half-Life and Half-Life 2, and its story largely stands on its own through Chell, GLaDOS, and the Aperture Science Enrichment Center. References to Black Mesa and the wider world help establish that the games share continuity.

The other options are also Valve series, which makes them tempting choices, but Counter-Strike, Team Fortress, and Left 4 Dead are not the series Portal is canonically identified as sharing a universe with. Portal began as an experimental project and was originally included in The Orange Box alongside Half-Life 2: Episode Two and Team Fortress 2.