The correct answers are Were aliens to land, If aliens were to land, and If aliens landed.

This is a second conditional sentence (unreal present/future), indicated by "would be required" in the main clause.

To express a highly unlikely hypothetical event, we can use "if + subject + simple past" (If aliens landed) or "if + subject + were to + base verb" (If aliens were to land).

For a more formal, literary tone, we can drop "if" and invert the subject and "were" (Were aliens to land).

"Had aliens landed" is incorrect because it is a third conditional form, which mismatches the main clause.