Alexandra and her best friend had a fight, after which they didn't speak for months. then alexandra came across a ticket stub from a concert the two girls had attended together, and suddenly she recalled happy memories of times she spent with her friend. she then decided it was time to call her friend and try to make up. how did the ticket stub alexandra access these memories?

It served as a retrieval cue that was encoded with the other happy memories.

Explanation:

Retrieval cue is something that can trigger the retrieval of information that we already stored in our Brain.  In most cases, retrieval cue will be something that strongly correlated with the information or something that exist near us during the information storing process.

Let's examine the excerpt above.

- happy memories of times she spent with her friend

This is the information that Alexandra stored in her brain

-  Alexandra came across a ticket stub from a concert the two girls had attended together, and suddenly she recalled . . .

In this scenario, a Ticket stub is strongly correlated with the memory since it is the one that provide opportunity for Alexandra  to have fun with her friend

Because of this, we can conclude that Ticket Stub act as a retrieval cue that trigger Alexandra memories of the events that happened using that Ticket Stub.

i think its toxic gasses but i could be wrong