Definitely a business.

Rental activity qualifies as a business or just an investment?

     Investment:  Investors are passive—they put their money in someone else's business
and hope their investment will increase in value due to the other person’s
efforts.

     Business:  business owners earn their profits by actively running a business,
either themselves or with the help of others they hire, such as managers.  Owning rental property qualifies as a business if you do it to earn a profit and work at it regularly, systematically, and continuously.

"Every Landlord's Tax Deduction Guide", 15th Ed., P 31