The correct answer is b. The explorers were climbing up the steeply sloping mountain.
Out of all the choices, the word climbing is used as a gerund in this option: b. The explorers were climbing up the steeply sloping mountain.
It was not used as gerund in the following: a. Mr. Herrera planted climbing ivy on the side of his house and c. The children enjoyed climbing on the playground equipment. I hope that this answer helped you.