Final answer:
The option 'mountain climber's decision to abandon his friend on the slope to find help would change the lives of both men' is the best answer for the underlined portion as it reflects the original sentence's meaning and is grammatically correct.
Step-by-step explanation:
The best answer for the underlined portion of the following sentence 'The mountain climber's decision to abandon his friend on the slope to find help would change the lives of both men' is: 'mountain climber's decision to abandon his friend on the slope to find help would change the lives of both men.' This option closely mirrors the original sentence while maintaining its clarity and grammatical correctness.