The line from Chapter One of Jack London’s The Call of the Wild that contains dialect is "You ain't going to take him out now?" the driver asked. Option B is correct.
Ain't is short form of am not, is not, are not, has not, or have not. It is a dialect, typically for the south and not used in proper English.
A dialect is a form of the language that is spoken in a particular part of the country or by a particular group of people. There are many different dialects of English and they have different words and grammar.