By definition, Average Total Cost equals Average Variable Cost plus Average Fixed Cost: \(ATC = AVC + AFC\).
Rearranging this gives \(AFC = ATC - AVC\), which is exactly the vertical gap between the ATC and AVC curves at any given output level.
As output rises, this gap narrows because fixed costs get spread across more units, though it never fully closes to zero, and it measures average fixed cost, matching option 1.