Step 1: Understanding the Concept:
The question asks to identify the primary and most dominant medium for advertising in the contemporary era. This involves understanding current trends in marketing and media consumption.
Step 2: Detailed Explanation:
In the 21st century, there has been a significant shift in advertising budgets from traditional media (like radio, television, print) to digital platforms.
The 'Internet' is the broad, all-encompassing medium for digital advertising. It includes advertising on websites, search engines (like Google), social media platforms (like Facebook and Instagram), and video streaming services (like YouTube).
While 'Mobile' (C) is a massively important channel, it is a subset of the 'Internet'. Mobile advertising refers to ads delivered to smartphones and tablets via the internet. Since the internet covers advertising on mobile, desktop, and other connected devices, it is the more comprehensive and accurate answer.
Radio (A) and Public Contact (D) are still used but are not the main source of advertising by a large margin. Global advertising expenditure data consistently shows that internet advertising holds the largest market share.
Step 3: Final Answer:
The Internet is the main and most comprehensive source of advertisement at present.