Assertion :All minerals are ores.
Reason :Ores are minerals from which metal can be extracted conveniently and econoically.

All the ores are minerals but all minerals cannot be ores. Because metal may occur in a number of minerals but every mineral is not suitable for the extraction of the metal. This is because the mineral may contain a low concentration of metal or may contain a large concentration of impurities which may not be easy to remove or process of extraction may not be feasible or commercially viable.  

Hence the assertion is False. 

The minerals from which the metals can be conveniently and economically extracted are known as ores.

Hence the Reason is True.

Hence the correct option is 'C'.