If this is homework, you should get the answer from your notes as it's a matter of debate.
Large organisations (or smaller ones with deep pockets) seem to prefer oracle, and alternatively m$ sql server. The free options are mysql and postgres; mysql is kinda dated yet faster and tailored around catering to web based applications, whereas postgres follows the standards closely (possesses many features, mayhap on par with oracle). Mongodb is the main contender in the nonrelational / nosql area, stores unstructured data and supports cloud storage.
Plenty of others which are order of magnitude smaller, fighting their way to recognition, but notably access seems to be used as a tutorial db; and sqlite might just be the most ubiquitous of the bunch, despite what the numbers say. It gives decent sql & db storage capabilities for pretty much any language, with the limitation of single user access. As such, it's more of an embedded data structure than management system. You can find half a dozen instances in all the major browsers and most operating systems, for example.