The slow conversion of dead plants into coal is referred to as 
 

Concept: The process of slow conversion of dead plants into coal .
is referred to as carbonization.)After plant dies,micro-organism acts upon dead plants and later layers of mud and other plant material settled upon them.These accumulated layers create high temperature and pressure underneath earth and 
Converts into coal.Carbonization is the slow conversion process of plants into coal.
Hence,the correct option is 1