Read properly, the stem says:

In the stimulus, there will be something notable and weird about the way Popkin arranges her flights. The correct answer will provide an explanation for why she structured her flights in such a specific, weird way.

In other words, the fact that this is an explain question tells us that something in the stimulus will demand explanation. Going into the stimulus with that framing makes catching the quirks in Popkin’s flight plans and anticipating the correct answer much more achievable.

Here’s a summary of the stimulus…

Popkin lives in Toronto. She’s gonna spend most of the work week in Montreal (Tu - Fri) and return home for the weekends (Sat - Mon). A sensible default option would be to buy a bunch of round-trip tickets that start and end in Toronto. But Popkin isn’t doing that. Instead, she’s jumping through some hoops to structure most of her tickets as round trips in the other direction – with Montreal as her “home base.”

…and an anticipation to bring into the answer choices:

The correct answer will provide some reason why Montreal makes a better “home base” than Toronto for round-trip tickets.