An email contains a textual birthday greeting, a picture of a cake and a song.…

The correct content-type is multipart/parallel. This type is used when an email contains distinct parts (like text, images, and audio) that can be displayed in any order or simultaneously. Multipart/mixed requires a specific display sequence, while multipart/alternative is used for redundant versions of the same content (e.g., plain text and HTML). Since the question states the order is not important for distinct media types, multipart/parallel is the appropriate choice.