The sentence that best exemplifies the theme of a climbing the mountain symbolizing overcoming obstacles is A: a high school student learns an important lesson when she abandons a good friend in the time of need.
This choice suggests a journey or challenge (the lesson learned), which aligns with the metaphor of climbing a mountain as a representation of overcoming obstacles. However, it's worth noting that it doesn’t explicitly reference climbing a mountain.
The other options either discuss themes of love, character conflict, or good vs. evil, which do not directly align with the theme of overcoming obstacles represented by climbing. If you are looking for a more direct example of a mountain climbing theme, you might want to adjust the choices to specifically mention a mountain.