The process of changing the one chemical substance to another chemical substance is called as chemical reaction.
The given chemical reaction is,
The number of parts of ethane required when the number of oxygen are 7 parts are 2.
When oxygen is 21 parts, then it can be written as follows:
Hence, the number of ethane required are parts of ethane.
Hence, the required part of ethane is 6.
Hence, the correct option is.