Final answer:
The mob heads of Zombies, Creepers, and Skeletons can be obtained by killing the respective mob, finding them in chests, or trading with villagers. So, the correct answer is 1) By killing the respective mob.
Step-by-step explanation:
The mob heads of Zombies, Creepers, and Skeletons can be obtained in a few different ways.
Killing the respective mob: When you kill a Zombie, Creeper, or Skeleton, there is a chance that they will drop their head as a rare drop.
Finding them in chests: You can also find mob heads in various chests throughout the game, such as dungeons, temples, and mineshafts.
Trading with villagers: In some versions of the game, you can trade with villagers to obtain mob heads. However, this method is not available in all versions.
Mob heads cannot be crafted using specific materials, so crafting is not a method to obtain them.
Thus, the correct answer is 1) By killing the respective mob.