To mark each Account in a List<Account> as either Active or Inactive based on the LastModified field value being more than 90 days, the developer should use a for loop, with an if/else statement inside. This technique allows the developer to iterate over the list of accounts, check the condition for each account, and update the field value accordingly. A switch statement is not suitable for this scenario, because it can only compare one expression with a set of constant values, not a range of values or a date calculation. An if/else statement without a for loop would not be able to process the entire list of accounts, only one account at a time.
References:
* : Loops
* : Switch Statements
* : If-Else Statements