Step 1: Note what the dentist needs to see.
A cavity is tiny and sits at the back of the mouth, so the dentist needs an image that looks bigger than the real tooth, viewed from close up.
Step 2: Rule out mirrors that cannot magnify.
A plane mirror only ever shows a same size image, and a convex mirror only ever shrinks the image, so neither one is useful here.
Step 3: Use the property of a concave mirror held close to an object.
When an object sits between the pole and the focus of a concave mirror, the mirror produces a virtual, erect image that is magnified, exactly what is needed to inspect a small cavity clearly.
\[ \boxed{\text{Concave mirror}} \]