What type of software generates revenue by displaying advertisements?

Adware is a type of software specifically designed to generate revenue through the display of advertisements. It typically delivers ads to the user while they are using the software or while they are browsing the internet. The primary business model behind adware is to collect revenue from advertisers who wish to promote their products or services, making it a common method of monetizing software applications, particularly those that are provided for free.

In contrast, the other options listed do not focus on generating income through advertisements. Abstraction refers to a programming concept that simplifies complex systems by allowing the user to work with a model or representation rather than with all the details. Alpha testing is a stage in software development where the application is tested internally before being released to a wider audience. Anti-virus software is designed to detect, prevent, and remove malicious software but does not generate revenue through ads. Therefore, adware stands out as the only option that aligns directly with the revenue generation model based on displaying advertisements.