From the Super Mario Wiki
Rivets (called bolts in NES Remix) are items that Mario must destroy in level 100m of Donkey Kong. There are eight (six in the Game Boy version) yellow rivets that are worth 100 points each. Mario must simply walk, run, or jump across a rivet to remove it. After all the rivets are gone, the platforms supporting Donkey Kong will collapse, allowing Mario to rescue Pauline. Removing a rivet also creates a gap that Firefoxes cannot cross. If a rivet is removed while a Firefox is on top of it, the enemy will become stuck over the gap. This allows Mario to safely jump over the enemy repeatedly to gain bonus points. Points are the only way to gain extra lives in this game.