Let's think about what kind of software or service is named Opera in the world of the internet. The question gives four possible categories: a conference site, an advertising firm, webinar software, or a browser.
Opera Software was a company based in Norway, and its main product, also called Opera, is a web browser, which is a program used to open and view web pages. Around the time of this exam, in 2008, Opera was already a popular alternative to browsers like Internet Explorer and Firefox, and it was known for being fast and light on system resources.
Opera has no connection to hosting web conferences or webinars, and it was never in the business of selling web advertising, so those two options can be ruled out right away. A conference site is a website built for meetings, which is also not what Opera is.
Once those three unrelated categories are ruled out, only the browser option remains, and that matches what Opera actually is.
So the correct answer is option 4, Web browser, because Opera is a browser application used to access the internet.