What is a unique feature of the Portrait ad format?

The Portrait ad format's unique feature lies in its inclusion of interactive modular apps. This allows advertisers to create more engaging and dynamic ad experiences that can capture the attention of users and provide them with interactive elements, thus enhancing user interaction and experience with the ad itself. By incorporating interactive modular apps, advertisers can deliver content that encourages users to engage with the ad, such as tapping for more information or navigating through different options, which can lead to higher retention and conversion rates.

The other options do not accurately represent the characteristics of the Portrait ad format. For example, the format is not limited to playing sound exclusively, nor is it restricted solely to video ads. Additionally, it does not lack user interaction capabilities; in fact, the presence of interactive features is a defining aspect that differentiates it from more passive ad formats.