What visual aid shows the proper approach angle during landing?

The Visual Approach Slope Indicator (VASI) is the correct answer as it provides pilots with critical information about the correct approach angle during landing. This system conveys whether the aircraft is on the proper glide path relative to the runway. The VASI uses lights that change color to indicate whether the aircraft is above, below, or on the designated glide slope, assisting pilots in achieving a safe landing approach.

The other options, while relevant to runway operations, do not specifically indicate the approach angle. Runway Edge Lights are primarily used to define the edges of the runway at night or during low visibility, and Threshold Lights provide visual cues at the beginning of the runway but do not assist in determining the glide path. Tie lights, while used on some runways, serve different purposes and do not pertain to approach angles specifically. Thus, only the VASI fulfills the function of guiding pilots on their approach slope.