= Solution
An isochrone is the locus of stars of a common age and composition over a range of initial masses. A real cluster diagram approximates it, with scatter from binaries, <Measurement error>[measurement errors], composition spread and differential reddening. Region A is the hydrogen-core-burning <main sequence>. Region B contains evolved subgiant/red-giant stars, typically with hydrogen-shell burning around an increasingly helium-rich core. Region C is the <horizontal branch>, occupied by core-helium-burning stars; RR Lyrae lie in its <instability strip>.
Point D is the <main-sequence turnoff>: core <hydrogen> exhaustion drives structural readjustment toward shell burning and expansion. Point E is the tip of the red-giant branch, where a low-mass star ignites <helium> in its degenerate core, the <helium flash>, and subsequently moves toward horizontal-branch conditions. The two labeled events are different evolutionary transitions, not merely changes in apparent color from dust.
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