Answer:
A. Many people who like to go hiking have spent a weekend roaming Yellowstone Park.
Step-by-step explanation:
The correct answer is A due to the fact that who in this case is referring to people, but not any kind of people, it is talking about the people that likes to go hiking. Who like to go hiking is a defining relative clause, this kind of clauses are not preceded by a comma and always are making reference to the previous noun (in this case people). The aim of who is narrow down the scope of options making reference to a certain group of people (the ones that enjoy hiking). If Who likes to go hiking is between commas that would be extra information that is not needed in the sentence to understand it, which is not the case here.