which is correct?

[udara sankalpa]

Dear Teachers

Kindly let me know which following sentence is correct.

-Sara is to study companies' histories.
-Sara is to study companies' history.

Kind Regards
Udara

[Tdol]

If it's a subject, I'd use 'company history', but if it is not, I would use 'companies' histories'.

[udara sankalpa]

Dear Tdol

I am delighted that you have responed to my thread.

Well it was not the subject. In fact, this came in my IELTS listening test held the day before yesterday. I put "companies' histories" but was kind of worried how it might be marked. Will I get the point there?

Thank you most sincerely again for your respose.

Kind Regards
Udara