What is the first step to destructively change the saturation of an image on a layer?

To destructively change the saturation of an image on a layer, utilizing the option for Hue/Saturation under the Image adjustments is the preferred approach. This option directly modifies the pixel data of the layer itself, leading to a permanent change that affects the saturation levels within the image. When this method is employed, any adjustments made to the saturation will overwrite the original pixel information, resulting in a final output that reflects those changes.

While other options might present methods for altering color parameters, they either apply adjustments in a non-destructive manner (like adjustment layers) or do not pertain directly to saturation adjustments. Specifically, non-destructive methods allow for later modifications or reversion, which does not meet the requirement of a destructive change.

The direct access to the Hue/Saturation adjustment through the Image menu facilitates immediate pixel manipulation, making it the correct choice for achieving a permanent saturation adjustment on the image layer.