Which of the following correctly completes the sentence?
according to the policies, staci is allowed to take the requested amount of vacation time.
corporations's
corporationes
corporation's
corporations

I am pretty sure that  the third of the following answers represented above is the option that correctly completes the sentence. Because we have the word ''company'' which is singular noun (so we can immediately omit the first option {corporations} and the last is the plural form {corporations}. Then the second option because it is simply spelled improper. So, what we have is corporation's with right usage of possessive 's'.
The sentence must look like that:
According to the corporation's policies, Staci is allowed to take the requested amount of vacation time.

The correct answer that would complete the sentence corporation's. According to the corporation's policies, Staci is allowed to take the requested amount of vacation time. The correct answer is the word corporation with the 's because this shows possession or ownership. The owner of the policies is the corporation. Since the word corporation is in the singular form, it takes the 's.

That's something your gonna have to yourself with buddy!