Step 1: Understand the question's focus.
We need to identify the primary role of an intranet in a business, distinguishing it from the broader public Internet and other networks.
Step 2: Break down the term "intranet" etymologically.
The prefix "intra-" comes from Latin and means "within" or "inside." An intranet is therefore a private network that exists inside an organization, accessible only to authorized employees and personnel.
Step 3: Eliminate option (A) - Selling products online.
Selling products online is e-commerce and takes place on the public-facing Internet, where any member of the public can visit the website. An intranet is private and not publicly accessible.
Step 4: Eliminate option (C) - Advertising on social media.
Social media platforms are external, public tools. Advertising on them requires the public Internet, not an internal private network.
Step 5: Eliminate option (D) - Exchange of information outside the organization.
Sharing information with external parties such as suppliers, partners, or clients uses the public Internet or an Extranet (a semi-private network giving selected outsiders limited access), not an Intranet.
Step 6: Confirm option (B) - Electronic exchange of information within the firm.
An intranet's core purpose is to serve as a secure, private platform for sharing company documents, announcements, HR policies, internal directories, and collaboration tools exclusively among employees inside the organization.
\[ \boxed{ \text{To enable electronic exchange of information within the firm} } \]