From the exam perspective I would expect the questions to be more around Table 2-1 (Pg. 22) in the PMBOK where they will reference a type of organization and/or show you a picture similar to Figure 2-1 and ask you which type of organization you are looking at or is being described. Therefore you need to be able to determine a weak vs strong matrix vs. Funtctional vs. balanced based on the scenerio they give you.

Where are you seeing in the PMBOK that it says to assume functional? I can't seem to find it to better understand your reference?

Per PMBOK(5th version), unless otherwise specified, we should assume Organization to behave like FUNCTIONAL ORG. However from Human Resource Management perspective, most of the places/blogs/exam simulators suggest to assume it as MATRIX ORG.

Which one is correct [from PMP exam perspective] ???
Or both are correct - one from general perspective and other from HRM perspective ?

Please justify.

Thanks & regards.
Subhajit