Answer:
Array: (a) All the messages a user has sent.
Variable: (b) The highest score a use has reached on the app. (c) A username and password to unlock the app.
Step-by-step explanation:
An array generally has more than one value whereas a variable can only contain a single value at any particular point in time. In addition, a variable has a limit whereas an array does not have any maximum limit. Therefore, it can be concluded that option (a) will be stored as an array while options (b) and (c) will be stored as variables.