Final answer:
To create soft edges on a selection, the feather tool is the appropriate choice as it provides a gradual blend between the selection and surrounding pixels. Other tools like the blur, eraser, and smudge tools serve different purposes and may not achieve the same soft edge effect.
Step-by-step explanation:
When you want to soften the edges of a selection in image editing software, the best tool to use is the feather tool. Feathering smooths out the selection's edges by creating a gradual blend between the selection and its surrounding pixels, giving a soft edge that integrates more naturally with the rest of the image. In contrast, using the blur tool might influence a larger area than desired, and it's more for general blurring of an area in an image. The eraser tool typically removes pixels and would not provide the soft edge you're looking for. Lastly, the smudge tool is better used for smearing or blending pixels together, which can be useful for certain effects but less so for creating a soft-edged selection.