The subject "team" is singular, and therefore the possessive pronoun should also be singular. The word "their" is plural, so it needs to be replaced with the singular possessive pronoun "its."
The corrected sentence is:
The team, on its expedition to the remote and unforgiving wilderness, faced countless challenges, from treacherous terrain to unpredictable weather.
Step 1: Conclusion.
The underlined word that needs to be changed is "their" in option (A).