Answer: In order for a substitution reaction to take place, at a minimum, loss of a leaving group and nucleophilic attack are needed.
Step-by-step explanation:
When one group in a molecule is replaced by another one then it is called a substitution reaction.
For a substitution reaction reaction to occur at a minimum, it is necessary to have a leaving group that helps in creating a carbocation and hence a nucleophile can easily attack.
For example,

Thus, we can conclude that in order for a substitution reaction to take place, at a minimum, loss of a leaving group and nucleophilic attack are needed.