A large program has been written for a company which rents out properties to tenants for long or short periods.
There are separate data files which hold information about
A the tenants
B the properties
C the payments made by tenants
D the payments made by the company for the maintenance of the properties.
What will the programmer need to do and think about when designing test data for such a system?

I thought the question was clear (and coincidentally straight forward). What part arn't you understanding?

All but the first and last lines are irrelevant to the question, actually. Apart from some implementation details the question has a universal answer that's applicable to all systems.

I would think about how I would slap the designer(s) in the head for this architecture

Any one want to suggest using SQL and linked tables ?

(Note: Python 3 now comes with SQLite ... and that might be a nice student way to begin a program to handle these linked data 'Tables' ?)