Answer: The answer is "the act of implanting a convention or norm into society".
Explanation: Institutionalization is a process where a certain convention or norm is embedded or implanted withing a social group or society in general.
For example,
1- The institutionalization of democracy in any given society.
2- A family that follow certain rules or conventions, or behaves in a certain manner around finances, food, religious believes, etc.
In conclusion, the definition that best defines the term institutionalization is "the act of implanting a convention or norm into society".