Hello,
I have to define a set of email addresses that depends on a workflow approval.
I also decided to implement this functionality in an external xlsx file, like the Config one.
eg. First row: 1. Flow1 | a@abc.com, b@abc.com, c@abc.com
Second row: 2. Flow2 | a@abc.com, c@abc.com
Third row: 3. Flow3 | b@abc.com, c@abc.com
Which activities do you think are the most suitable in order to implement this functionality?
Activities you intend to use depend on the infrastructure your org is using
obviously you need to use UiPAth.Mail.Activities and if your org is using a exchange server link for
mail automations us can use Exchange mail activities, if it is outlook you can use outlook activities etc more info here 
UiPath docs
to read config file you can use workbook activities if office is not installed, if office is installed yu can use excel activities
hope this is helpful