Which military time signifies the start of a new day?

The military time that signifies the start of a new day is 0000 hrs, which represents midnight. In military time, each hour is denoted in a 24-hour format, beginning from 0000 hrs at midnight and ending at 2359 hrs just before the next midnight. Therefore, 0000 hrs marks the exact point when one day transitions into the next, making it the correct answer.

The other times provided refer to specific moments during the day rather than the beginning of a new one. For instance, 0600 hrs is early morning, 1200 hrs represents noon, and 1800 hrs corresponds to 6 PM in the evening. None of these times indicate the start of a new day like 0000 hrs does.