This question comes from a quiz but I could not find the correct answer.
This image comes from a Quiz "Which English?" which people were invited to take as part of a study into language acquisition. While the Quiz purports to be able to identify which local variant of English you speak from your answers, it actually serves dual purpose and forms part of a study into how language acquisition varies with age.
According to the supplementary materials (see p.58) of the paper for which the Quiz was conducted, the correct answer varies by English region so there is no universally correct answer to this question.
There is no way to know. As mentioned in a comment, using the indefinite article "a" does not mean that the hikers could not have climbed the same hill.
"There was an event at the park. Each participant crossed a stream, climbed a hill, and rode a bike 5 kilometers."
There is only one stream and one hill in the park. And of course, there was more than one bike -- each person had his own bike, because, really, what else would make sense? Sometimes, context is all, and language is not math. The sentence is grammatically and factually correct. However, it's possible that there was one bike and they took turns riding it. That would be something that a scientific paper, legal brief, or other technical documentation would have to clarify. In creative writing, that kind of precision can kill the flow and tone.
The indefinite determiner "a" represents all things in a given category rather than the more specific definite determiner "the". So, if you were to ask me, "Every Hiker climbed a hill" represents a specific group of hikers who climbed non-specific hills and not the same hill. If the sentence said, If every hiker had climbed the hill, it would be only one hill.
This seems quite clear to me. In the first picture, every climber climbed a hill; in the second picture, every climber climbed the hill. It is easy to construct counter-examples where the second picture is described as "every climber climbed a hill", but the default meaning is represented in the first picture. This is why the question is "Which image is most correct?"
In my native dialect (General American), it’s ambiguous, but I’d assume it probably meant multiple hills. “Each hiker climbed a hill,” makes that presumption even stronger. “Every hiker climbed the [same] hill,” and “Every hiker climbed his [own] hill,” are both unambiguous.