"Principle" or "Principal"?

Solution 1:

From merriam-webster.com:

Principle:

Usage discussion:

Although nearly every handbook and many dictionaries warn against confusing principle and principal, many people still do. Principle is only a noun; principal is both adjective and noun. If you are unsure which noun you want, read the definitions in this dictionary.

In the exercise from the book, the word in question is an adjective modifying "goals," hence it must be principal. As for whether principal (the noun) can refer to more than just people, the dictionary is clear:

1: a person who has controlling authority or is in a leading position

2: a matter or thing of primary importance