Tower visibility observations must be taken and reported when either the reported surface and/or tower prevailing visibility is less than __ statute miles.

Tower visibility observations are an essential component of aviation weather reporting, particularly for ensuring safe operations around airports. The standard stipulation for taking and reporting tower visibility is indeed when the reported surface and/or tower prevailing visibility is less than 4 statute miles. This requirement is crucial because visibility conditions below this threshold can significantly impact pilots' ability to navigate safely, especially during takeoff and landing procedures.

The emphasis on 4 statute miles ensures that pilots are continuously provided with the most accurate and timely information regarding visibility, allowing them to make informed decisions based on current weather conditions. If visibility falls below this measurement, it triggers the need for updated reports, which can influence flight operations and air traffic management.

Therefore, the correct answer reflects a key guideline in aviation safety concerning visibility reporting protocols, highlighting the importance of maintaining operational safety standards in adverse weather conditions.