Some reptiles, like snakes, have olfactory receptors
on their tongues,
on the roof of their mouths,
on their skin,
at the tip of their tails

The correct answer for the fill in the blank is- On the roof of their mouths.

Some reptiles like snake possesses Vemeronasal organ ( a sense organ used for smell) that is also termed as Jacobson's organ. It is located in the nasal cavity that is present just above the roof of the mouth. It has olfactory receptors that detect specific organic compounds from their surroundings ( such as sex pheromones from their mating partners). They help in producing appropriate behavioral response in them.

option c is the correct answer.