Concept:"Break into" means to interrupt or enter something, such as a conversation, without being invited.
Explanation:In the sentence, children are interrupting the conversation of adults.
We need a phrasal verb that means "to interject" or "to join unasked."
Let us review each option.
"Break up" is used when a fight, meeting, or relationship ends or disperses.
"Break down" describes a machine stopping or a substance decomposing.
"Break off" means to snap or to stop abruptly, such as ending an engagement or breaking a piece.
None of these meanings match the act of intruding on a discussion.
On the other hand, "break into" can be used to mean entering a place without permission, but it also means interrupting a conversation.
For example, "He kept breaking into our talk whenever we paused."
Therefore, the correct option is C.
Answer:C. break into