The correct sequence for descending order of machinability is

Machinability refers to the ease with which a material can be cut or machined to the desired surface finish and accuracy.

Grey cast iron has the best machinability among the three because the presence of graphite flakes in its structure acts as a lubricant and reduces tool wear.

Low carbon steel has moderate machinability. It does not have graphite, so cutting is tougher compared to grey cast iron.

Wrought iron has the least machinability due to its fibrous structure and slag inclusions, which make cutting more difficult.

Thus, the descending order of machinability is Grey cast iron > Low carbon steel > Wrought iron.