Which of the following ad units has an initial dimension of 300×250?

The Medium Rectangle is a well-established ad unit in digital advertising that is recognized by its dimensions of 300×250 pixels. This size is commonly used in various placements on websites and is known for its effectiveness in displaying ads because it fits well within a variety of content and layout structures. The dimension allows for a good balance of visibility and interaction without being overly intrusive, making it a preferred choice for advertisers looking to engage users.

In contrast, Rich Media refers to a broader category of ad formats that can include additional interactive elements, but it does not specify a fixed size like the Medium Rectangle does. Native Ads focus more on blending seamlessly with the content of the platform but again do not have standard dimensions. Rising Stars are a set of creative ad formats developed by the IAB but are not specifically defined by the 300×250 size. Therefore, the Medium Rectangle stands out as the correct answer due to its specific and widely recognized dimensions.