Improve the bracketed part of the sentence.
Emptying (half the Earth of it’s) humans is the only way to save the planet.
It’s means it is or it has e.g. “It’s a nice day”. The possessive form is “Its” e.g. “The cat hurt its paw”.
So only option 2 or 3 can be the answer. But when the Earth is spoken of as a physical body or as a planet, occupying space-time, it's often capitalised and takes the article “The” before it. So, option 2 is eliminated. Option 3 is the correct answer.
Thus, the correct sentence is: Emptying half the Earth of its humans is the only way to save the planet.