Which action is considered "breaking in"?

The action classified as "breaking in" refers to the act of gaining unauthorized access to a property through means that typically involve circumventing security measures. Picking a lock exemplifies this because it involves manipulating the lock mechanism to gain entry without proper authorization or the use of a key. This method of entering a property is recognized as a criminal act, often associated with burglary, because it demonstrates intent to trespass and bypasses the owner's security measures.

In contrast, opening a door with a key is a legitimate means of entry, as it implies that the individual has permission from the property owner. Walking through an open door suggests that access is already granted, and therefore does not constitute breaking in. Similarly, knocking to gain entry involves seeking permission to enter, which also aligns with lawful behaviors. Overall, the correct answer reflects the illegal nature of gaining entry through deceptive or unauthorized means.