This would be C. the Real Estate Agents. The engineering firms are only in charge of the construction of the home and the materials that are to be used for building it. The Architects, on the other hand, are responsible for the design of the home and the placement of the materials of the home. The lending institutions are focused more on providing the home inspector with money that he will need for a piece of property. So that leaves us with the Real Estate Agents. It is these people who deal with the sales of property which is why they are the source of business leads.
That would be known as taxable income