Final answer:
Rooney Concrete Company primarily engages in pouring concrete slabs for single family homes, which is the main activity of the business.
Step-by-step explanation:
The main activity of Rooney Concrete Company as stated in the question is pouring concrete slabs for single family dwellings. This indicates that their primary business operation is in the construction sector, more specifically, in the creation of foundational structures for homes. As such, the correct answer to the question is option 3, pouring concrete slabs. It does not concern manufacturing furniture, building bridges, or designing software, which are unrelated to the company's stated primary activity.