Explanation

(c)
(i)enantiotropy when two forms of a solid substance exist together in equilibrium with each other at a particular temperature under normal pressure e,g,
S_{R}⇌S_{M}
(ii)dynamic allotropy if different allotropic forms exist in equilibrium over a range of temperature.
(iii)monotropy if an allotropic form change slowly to a stable form e.g.,
O_{3}⟶O_{2}
$\therefore$ Monotropy is correct answer.