Historians label an event as a turning point when it: a. marks the beginning of a distant historical period. b. does not fit the major themes of a historical period c. affects more than one region during a historical period d. occurs repeatedly throughout a particular historical period

The answer is Marks the beginning of a distant historical period.
For example, the tragedy of 9/11 could be considered as a turning point because it becomes the reason for Government agencies to start various cyber security programs to monitor potential terrorist activities that endanger the safety of the nation's land (which was leaked by edward snowden)

Ifound this on google”the munich conference of 1938 was prompted by d. hitler's intention to take control of part of czechoslovakia. hitler sought to unite german-speaking people in a single nation and thus turned his sights on the sudetenland, a german-speaking region of czechoslovakia.” i hope it you