The correct answer is a) rockfalls.
Cho sees a talus while hiking. She should be careful of rockfalls.
A talus is an acumulation of rocks or debris in a slope on a hill or a mountain. Cho is hiking, not climbing, that is why she has to be careful of rockfalls when she sees a talus. If she was climbing, due to the height of the mountain she could be careful or glacier melting and rockfalls. But the questions says clearly she is hiking so the correct answer is rockfalls.