The correct answer is option 'B' - A.M.

Explanation:
The terms A.M. and P.M. are used to distinguish between the two halves of a day. A.M. stands for ante meridiem, which is Latin for "before noon," while P.M. stands for post meridiem, which is Latin for "after noon."

The first half of the day, from midnight to noon, is referred to as A.M. This time period includes the early morning hours, such as 1 A.M., 2 A.M., and so on, up until 11 A.M. At 12 noon, the first half of the day ends, and the second half begins.

The concept of dividing the day into A.M. and P.M. allows for a clear and consistent way of expressing time. It helps to avoid confusion and ensures that everyone understands whether a particular time refers to the morning or the afternoon.

Other options mentioned in the question, such as P.M., A.D., and B.C., are not applicable to the first half of the day.

- P.M. (post meridiem) refers to the time from noon until midnight, which is the second half of the day.
- A.D. (Anno Domini) is used to indicate years in the Gregorian calendar and has no direct relation to the time of day.
- B.C. (Before Christ) is a designation used for years before the birth of Jesus Christ and also has no direct relation to the time of day.

In conclusion, the first half of the day, from midnight to noon, is called A.M.