Final Answer:
The block that spawns mobs is the 4) Monster spawner block.
Step-by-step explanation:
While other options like "Mob generator block" or "Creature spawner block" might seem like potential answers, they are not official terms used in the game. The Monster spawner block is the only established and recognized block specifically designed to spawn mobs.
These spawner blocks are cage-like structures found naturally in the world or generated within specific structures like dungeons. They continuously spawn mobs of a specific type, which can be identified by a miniature model of the mob spinning inside the cage. Breaking this block will only yield experience orbs, as it cannot be obtained as an item or moved by pistons.
Therefore, the correct answer is 4) Monster spawner block.