Final answer:
Before visiting the Clutter household, the two travelers stopped at a diner where Perry saw a black cat, which he considered a bad omen.
Step-by-step explanation:
The last stop made by the two travelers before they visited the Clutter household was a diner. At this diner, Perry, one of the travelers, perceived a bad omen which was seeing a black cat. Perry considered the sight of the black cat as ominous, suggesting a traditional association of black cats with misfortune or ill omens. Consequently, the correct answer to this multiple-choice question is option a)