Amalgam is used in extensive cavities.. When cusps lost and thin supported wall

Amalgam is used in extensive cavities:

• a- When the cusp is supported by dentine and proper retentive preparation. 
• b- When cusps lost and thin supported wall.***
• c- When one cusp is lost and need to apply restoration to replace

The correct answer is b- When cusps lost and thin supported wall.

Amalgam is a strong and durable filling material that is well-suited for extensive cavities. When cusps are lost or the tooth walls are thin, amalgam provides the necessary support to prevent the filling from fracturing or the tooth from breaking.

- Option a, "When the cusp is supported by dentine and proper retentive preparation,":

is not a suitable situation for amalgam because there is enough tooth structure to support other types of fillings, such as composite resin or glass ionomer cement. These materials are more aesthetically pleasing than amalgam and bond chemically to the tooth, which can provide a stronger seal.

- Option c, "When one cusp is lost and need to apply restoration to replace,":

is too specific and does not address the general principle of using amalgam for extensive cavities. Amalgam is a good choice when multiple cusps are lost or the tooth walls are thin, regardless of whether one cusp is lost.

Therefore, the best answer is b- When cusps lost and thin supported wall.