Which of the following is an example of "breaking in"?

The concept of "breaking in" typically refers to entering a premises or property unlawfully and often involves some form of forceful entry. When considering the options provided, breaking a window fits this definition well, as it implies a destructive method of gaining access, which aligns with the illegal act of breaking in. This kind of entry not only violates property rights but also demonstrates the intent to trespass unlawfully, thus encompassing the broader legal implications of breaking and entering.

In contrast, walking through a doorway or using a key to enter represents lawful methods of entry, assuming the person has permission to be on the property. Setting off an alarm may indicate an attempted entry or security breach but does not by itself provide a clear example of breaking in as it does not involve the act of entering through force or destruction.