The ranked mode that involves collecting and delivering clams is Clam Blitz.
In Clam Blitz, teams collect clams scattered around the arena and try to throw them into the opposing team’s goal. The goal is protected by a barrier, so players must first break it before depositing clams and reducing the opponent’s points counter.
Collecting 10 clams creates a Power Clam. Power Clams are used to break the opposing goal’s barrier, after which ordinary clams can be thrown into the open goal for additional scoring. This creates a tactical loop involving map control, clam collection, coordinated attacks, and defense of the team’s own goal.
Clam Blitz was the new ranked mode added in Splatoon 2. The other listed answers are also ranked modes, but their objectives differ: Rainmaker centers on carrying a powerful weapon, Tower Control requires moving a tower along a route, and Splat Zones focuses on controlling an inked area. Clam Blitz therefore has the single canonical answer for this question.