The sentences that use a gerund phrase as its subject are:
- Finding the treasure is foremost in the minds of the mutineers.
- Holding Jim hostage is part of Silver's strategy.
Both of these sentences have gerund phrases ("Finding the treasure" and "Holding Jim hostage") serving as the subjects.
The sentence "Discovering a human skeleton frightened the pirates" uses a gerund ("Discovering") but does not use a gerund phrase as its subject because "a human skeleton" is the direct object of the action.
So, the correct answers are the first two sentences.