What sort of treatment for molar cavity?

Small or large?: It depends on the size of the cavity. Small cavities can be easily filled with a tooth colored resin material while larger cavities may require porcelain inlays or crowns.

Depends on size: Very small cavities that are limited to the enamel can be cleaned out and covered with sealant. Larger cavities that have penetrated through the enamel will need a filling (silver amalgam or resin). Worst case scenario is a large cavity that has compromised a significant portion of the tooth which mandates a crown. Check with your dentist for options.

Depends: Depends on the depth and size of cavity (decay). Sometimes a root canal is needed. Other times just a filling/crown. If decay is too advanced - exctraction. If extraction - see an oral surgeon.