Final answer:
The correct answer is (a) Fatuous.
Step-by-step explanation:
The correct answer to the question is (a) Fatuous.
In the given sentence, the phrases 'someone with such a fatuous outlook on life' suggests that the person has a foolish or silly view of life. This would make them an unenjoyable traveling companion, especially on a long journey.
The word 'saturnine' means gloomy or morose, which does not fit the context of the sentence.
Therefore, the correct option is (a) Fatuous.