Challenge 1: Removing a Blemish From Subtly Varying Skin Lighting — Solution Walkthrough

The recommended tool:

The Healing Brush (or Spot Healing Brush, if a fully automatic source
is preferred).

Why, per this chapter's own material:

The Healing Brush automatically blends the sampled texture with the
surrounding area's own tone and lighting, rather than performing a
literal 1:1 copy — exactly suited to skin, where lighting genuinely
varies subtly across the surface. The blended result adapts to
whatever tone happens to be at the destination, producing a seamless
correction.

Why the Clone Stamp would be a worse choice here:

Per this chapter's own material, the Clone Stamp copies exactly —
texture, lighting, and shadow included, with no blending at all. Given
that lighting varies subtly across the skin, a direct 1:1 copy from
one spot to another risks pasting in lighting that doesn't match the
destination area, producing a visible patch that looks noticeably
different from its surroundings — exactly the kind of obviously wrong
result a literal copy tool produces when applied to a surface with
subtle tonal variation.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise is a direct, practical application of this chapter's own
Clone-Stamp-vs-Healing-Brush distinction to the specific scenario
(subtly varying skin lighting) the chapter itself names as the
Healing Brush's own ideal use case.