Mass communication is:

Concept — Every act of communication can be placed at a level, and three criteria decide which level it belongs to: who the source is, whether a technology carries the message, and how large and how anonymous the receiving audience is. Mass communication is the level at which all three are fixed together: the source is a formal organization, a technology serves as the medium, and the audience is large, heterogeneous and largely anonymous, receiving the message more or less simultaneously. Change any one of these three criteria and the act belongs to a different level.

Application — Test each definition offered in this question against those three criteria and keep the one that fixes all three at once. That definition is “Communication, when a source, typically an organization, employs a technology as a medium to communicate with a large audience”, which names the organizational source, the technology as medium, and the large audience together. This is the standard textbook definition of mass communication given by Baran and Davis in Mass Communication Theory.

Contrast — the remaining definitions are the same textbook’s names for the neighbouring levels of communication:

• “Communication between two or a few people, typically face-to-face” is interpersonal communication: the audience is a handful of people, no technology stands between them, and feedback returns turn by turn.

• “Communication between a few or many people that employs a technology as a medium” is mediated communication: a technology carries the message, but the source may be a single individual and the audience may be very small, so a two-person phone call satisfies it. Mass communication lies inside mediated communication as its organizational-source, large-audience end, which makes mediated communication the wider class rather than the definition of mass communication itself.

• “Communication with a group from within” is intra-group communication: the source belongs to the very group being addressed, so what fixes the level is the sender’s membership of the group, not a medium or an audience size.

Result — mass communication is communication in which a source, typically an organization, employs a technology as a medium to reach a large audience.