Explanation:

Sometimes it's hard for the dentist to get an accurate look at the teeth located in the back of the mouth (the molars). A concave mirror gives the dentist a magnified reflection of the mouth while also refracting a bit of light.

• Since dentists have to take a view inside our mouth for the purpose of seeing cavities or germs infection. So we need something to show the inner portion of teeth (molars) easily.

• Concave mirror - Concave mirror is that type of mirror in which reflecting surface is bent inward and outward surface is polished .These mirrors gives brighter and larger(magnified) image of the original object without inverting the image thus dentists can see larger image what is inside making the doctors work much easier in viewing inner teeth and the mouth .

Hence the correct option 3rd is correct.