What is the most likely explanation for a gurgling sound heard from a tub drain when the toilet is flushed?

The gurgling sound from a tub drain when a toilet is flushed is most likely caused by water discharging from the toilet pulling water from the tub trap. This phenomenon is linked to the way plumbing systems are interconnected; both the tub and toilet share a common drainage line. When the toilet is flushed, the surge of water creates a negative pressure in the drain system, which can lead to the water in the tub's trap being siphoned out, momentarily breaking the seal that prevents sewer gases from entering the livable space.

This is a direct result of the physics of fluid dynamics and drainage systems where interconnected lines can affect one another, particularly during high-flow events such as flushing a toilet. The sound you hear is the result of air rushing into the trap as the water is pulled out, which reflects the pressure changes occurring within the drainage system.

In contrast, the other options describe issues that wouldn't be the primary cause of the gurgling sound. A partially blocked tub drain could cause slow drainage or backups but is less likely to create a gurgling sound specifically attributed to toilet flushing events. Similarly, if the tub drain was pushing air due to a toilet flush, it would indicate a different issue where an obstruction might be causing unusual