The correct answer is (a) heated during metamorphism.
Radiometric dating is a technique used to date a material and to find the relative age of the material. When a rock is radiometrically dated then the age of rock is determined when the rock crystallized. The crystals of the rocks will roughly be of the same age. The radiometrically dated metamorphic rock determines the age, when the rock was heated during metamorphism. The main problem of this is that the rock is heated at a high pressure during metamorphism and because of this reason different crystals show different age.
