Answer:
The best option is C. A is reliable, B is unreliable, C is reliable.
Step-by-step explanation:
We can think of reliability as referring to how trustworthy the speaker is or how provable the information the speaker provides is.
In sentences A and C, the speaker states information that is provable. Even though sentence A uses the adjective "unpleasant" and "unfriendly", it is still a reliable sentence. Often, when using adjectives, speakers my be stating their own biased opinion. However, in this case, the notion of an unpleasant and unfriendly environment does not come from the speaker's opinion, but from the fact that pioneers had a hard time, encountering many challenges that required adaptation.
Sentence B, however, does not speak of a fact that is easy to prove. It is quite dangerous to affirm that none of the pioneers liked summer. It is not likely that any documents affirming such a thing exist. This is the kind of sentence that comes from bias and conjectures. Since the heat was new to the pioneers and they found it hard to adjust to, the speaker seems to have deduced none of them liked summer.