Identify the fallacy committed in the following argument: “There must be intelligent life on other planets. No one has proven that there isn't”.

The correct option is Appeal to ignorance.

An argument commits the fallacy of an appeal to ignorance (also known as argumentum ad ignorantiam) when it asserts that a proposition is true simply because it has not yet been proven false, or that it is false simply because it has not yet been proven true.

In this argument, the claim is: “There must be intelligent life on other planets.”
The premise supporting this claim is: “No one has proven that there isn't.”

Because the premise relies entirely on a lack of proof or evidence to the contrary (the "ignorance" or absence of negative proof) to establish the truth of the conclusion, it commits the appeal to ignorance fallacy.