Which of the following best defines e-commerce?

The definition of e-commerce encompasses a broad range of activities related to the buying and selling of goods and services over the internet. It involves not just digital products but physical products as well, and can include transactions conducted through websites, mobile applications, and online marketplaces. Therefore, the description of e-commerce as "the buying and selling of goods or services using the internet" captures the essence of various commercial activities that take place online, making it the most accurate and comprehensive choice.

The other options are more restrictive or specific in scope. Focusing solely on digital products limits the concept of e-commerce significantly, as it is not confined to those types of goods. Similarly, the use of social media for business purposes does not inherently involve any transaction or commerce, which is a key component of e-commerce. Lastly, the online distribution of digital marketing materials may be part of a business strategy but does not define the broader concept of e-commerce itself. Thus, the chosen answer provides the clearest and most inclusive definition of e-commerce.