Types of communication are distinguished by how many people are involved and how the message reaches them, so the two clues in the question - a single source and a large number of simultaneous receivers - identify the type directly.

Mass communication is the answer. In it one source sends a message through a medium such as television, radio, a newspaper or the internet to a large audience all at once. Three features are characteristic. The audience is large, and also scattered, since its members need not be in the same place. It is largely anonymous, in that the sender does not know the individual receivers. And feedback is delayed and indirect, arriving through ratings, letters, circulation figures or online responses rather than immediately. Because reaching such an audience requires technology, mass communication always involves a medium.

Group communication takes place among the members of a relatively small group, such as a committee or a classroom discussion, where members interact with one another and feedback is immediate. The audience is neither large nor anonymous.

Intrapersonal communication happens within a single person - thinking, reflecting, talking to oneself. There is no second party at all, so it cannot describe transmission to many receivers.

Interpersonal communication is the exchange between two people, or a very few, face to face or through a personal channel. Feedback is immediate and the relationship is personal, which is the opposite of the anonymity that marks the mass audience.

Hence the mode described is mass communication.