The correct option is B By
A preposition is a word which comes before a noun or a pronoun and establishes a relationship between the elements of a clause or words. Option B)by is correct as it is used to express how something is done. The other options are wrong as about is used to indicate a a connection with something, over is used to indicate a higher position, and across is used to express the other side of something. These uses don't fit in the given question. The correct answer is B)by.