I would say that the correct answer is D. The abbreviation "B.C." should be associated with a number, such as 44 B.C
In the case of A, it is not an abbreviation but a symbol, a representation of the current coin and it is not generally used.
B: You can use them interchangeably, you should choose one or the other.
C: the statement is false because you can use it alone. For example: "She got her Ph.D. in London "