One of the purposes of a colon ( is to introduce an item or a list of items. In this case, the item being introduced is her reason for beginning a career in law enforcement, which was “to answer a challenge made by her husband.” Without a colon, as in B, the sentence reads, “Beverly Harvard began her career in law enforcement in an unusual way to answer a challenge made by her husband,” so you have an introduction but no separation between the introduction and the item introduced. Without a colon, you should get rid of the introduction so that the sentence reads, “Beverly Harvard began her career to answer a challenge made by her husband.” I’ve heard that colons and semicolons are being tested a lot on the ACT these days. This is a good summary of colons: http://www.grammarbook.com/punctuation/colons.asp