Which underlined gerund functions as the object of a preposition? The valley later became a center of / mining / for borax. Because of the heat, frequent / resting / is advised. When you visit, avoid / hiking / in the midday sun. / Hiking / is now Death Valley's greatest tourist activity.

Prepositions are words that shows where the noun or pronoun is placed.

The valley later became a center of / mining / for borax. The object of preposition of mining here is borax.

Because of the heat, frequent / resting / is advised. The object of preposition of resting here is frequent.

When you visit, avoid / hiking / in the midday sun. The object of preposition here is hiking is avoid.

/Hiking / is now Death Valley’s greatest tourist activity. The object of preposition here is Death Valley.