The noun (Substantiv/Nomen) Multimedia is feminine in German. Therefore, the correct definite article in the nominative singular is die. So, you say: die Multimedia.
(multimedia)
Nouns ending in -a are usually feminine. Common in words of Latin/Italian origin
Examples:
Die Kombination von verschiedenen Medien wie Text, Bild, Ton und Video in einem System.
The combination of different media such as text, image, sound, and video in one system.
Multimedia ist aus der modernen Kommunikation nicht mehr wegzudenken.
Multimedia is an indispensable part of modern communication.