"What is the religion of that people?"
Translation:Quelle est la religion de ce peuple ?
"Un peuple" and "des gens" are different notions. In this sentence, "that people" refers to a group of individuals sharing a territory, customs, and possibly race (think of a tribe).
"These/those people" as an undefined group of individuals is "ces gens" or "ces personnes".
Was offered the flashcards to construct "quelle est la foi de ce peuple" (as well as "religion"). In my community the two words could be interchangeable. Should this be a valid answer and if not, why not?