Diamond ring is a phenomenon observed:

Answer: (c) only along the peripheral regions of the totality trail

The key picks this option.

The idea

The diamond-ring effect combines one bright glimpse of the Sun’s surface with the faint ring of its outer atmosphere during a total solar eclipse. Uneven terrain at the Moon’s edge lets the last or first sunlight pass through. It appears near the transition into or out of totality, rather than being restricted to a particular strip along the path.

Moon moves across the Sun’s disc → Most of the bright solar surface is covered → Sunlight passes through valleys at the lunar edge → One bright patch remains beside the faint corona → Totality removes that final patch → The effect can reappear as totality ends

Why the others are wrong

(a) The diamond ring appears just before totality begins and again as it ends.

(b) Also seen at the end.

The trap

The effect happens at both the start and the end of totality.

Also know

Baily’s beads are separate points of sunlight passing through lunar valleys. An annular eclipse leaves a ring of the Sun’s surface because the Moon appears too small to cover it. In a lunar eclipse, Earth’s shadow falls on the Moon instead. A solar eclipse requires a new moon and suitable alignment near an orbital node.