Link for the picture
The inflaton field can be represented as a ball rolling down a hill. During inflation, the energy density is approximately constant, driving the tremendous expansion of the universe. When the ball starts to oscillate around the bottom of the hill, inflation ends and the inflaton energy decays into particles. In certain cases, the coherent oscillations of the inflaton could generate a resonant production of particles which soon thermalize, reheating the universe.
When does reheating occur?
a) after inflation ends ; before the field reaches minimum
b) after the field reaches minimum of the potential
c) when inflaton oscillates around the minimum of the potential