Assuming your options are <em>infinitives, repetition, adjectives, and punctuation, </em>the correct answer is infinitives.
The answer is rather simple when you think about it - there are no infinitives in this passage, so obviously they cannot contribute to any mood. There are repetitions (the word fog is constantly being repeated), there are a lot of adjectives, and some punctuation marks - all of which contribute to the dreariness of this excerpt.
The answer to the question should be a: to indicate how excited he is about treasure hunting.
I believe that sentence 5 is the one where a semicolon should be added, as it joins two separate sentences of a similar content. It should look something like this: "<span>Water is not everywhere; it's miles away."</span>
