Answer:
There are four options for this question, A is correct.
A. Salesforce allows up to four objects in a single report type.
B. Salesforce does not support orders or orders line items.
C. A report cannot contain data from accounts and contacts.
D. Custom report types cannot contain opportunity data.
Explanation:
The Salesforce doesn't permit adding more than four objects if the user tries to add more than this limit, the user will receive an error message, this would be the main problem for an architect run into when implementing this in Salesforce, some reports need more data and with this limit is hard to do the reports.