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

The most likely explanation for a gurgling sound heard from the tub drain when the toilet is flushed is that water discharging from the toilet is pulling water from the tub trap.

When a toilet is flushed, a significant volume of water rushes through the plumbing system. This sudden influx of water can create a partial vacuum in the pipes downstream, which in turn can draw water from traps connected to nearby fixtures, such as the tub. The gurgling sound is an indicator that air is being displaced within the plumbing system as the trap is being affected by the flow of water from the toilet. This phenomenon is common in situations where multiple fixtures share a common drainage line.

The other scenarios presented do describe potential plumbing issues, but they do not directly account for the gurgling noise in the context of toilet and tub interaction. A partially blocked tub drain may cause slow drainage but wouldn't necessarily cause gurgling specifically linked to toilet flushing. Similarly, while a blockage downstream could lead to drainage issues, a clearly recognizable gurgling sound is more typically associated with the interaction of varying water levels in traps rather than obstruction alone.