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 appeal to ignorance (Latin: argumentum ad ignorantiam) is an informal logical fallacy that occurs when a proposition is asserted to be true simply because it has not yet been proven false, or false simply because it has not yet been proven true.

In the given argument:
“There must be intelligent life on other planets. No one has proven that there isn't.”
The arguer draws a positive conclusion (that intelligent life exists on other planets) based solely on the lack of evidence or proof to the contrary (that no one has proven it doesn't exist).
Because the premise relies on a lack of knowledge or proof, the argument commits the fallacy of an appeal to ignorance.