The correct answer is: E. No error
The sentence is grammatically correct and has no punctuation errors. The word “as” is used correctly to introduce a reason. The word “their” is used correctly to refer to the footprints of aliens. The word “would” is used correctly to express a future possibility. The word “last” is used correctly to mean “continue to exist.” The word “far” is used correctly to mean “to a great extent.” The word “longer” is used correctly to mean “for a greater period of time.” The word “than” is used correctly to compare two things. The word “radio signals” is used correctly to refer to signals that are transmitted through the air using radio waves.
The sentence could be improved by adding more details. For example, the sentence could say “The Moon may be the best place to look for aliens because their footprints on their surface would last far longer than radio signals. This is because there is no wind or rain on the Moon to erode the footprints.”