A.I. Explanation:

Option 1: "Its ability to fly at supersonic speeds"

Explanation: While the B-52 is described as a supersonic bomber, the text does not state that this feature contributes to its global reach capability. The focus is on its air-to-air refuelling capability as the key factor for reaching any point on Earth.

Option 2: "Its length of 156 feet and wingspan of 185 feet"

Explanation: The dimensions of the B-52 are mentioned in the text, but they do not relate to its global reach capability. The text emphasizes that the aircraft's ability to refuel in the air is what enables it to reach any location globally.

Option 3: "Its air-to-air refuelling capability"

Explanation: The text explicitly states that the B-52 can climb to 50,000 feet with air-to-air refuelling, which allows it to reach any point on Earth. This capability is crucial for its global reach, making it the correct answer.

Option 4: "Its ability to climb to 50,000 feet"

Explanation: Although the B-52 can climb to 50,000 feet, this feature alone does not provide the global reach capability. The text highlights that the air-to-air refuelling capability is what enables the bomber to operate globally.