Ahistorian who compares world events during the classical era to world events during the modern era is organizing history by . a. period. b. region. c. culture. d. theme

The answer would be period. This history by period sum up noteworthy periods in the history of the world, from the ancient world to the present day. The following are the periods:
•        Ancient history (60,000 BC – 650 AD)
•        Postclassical Era (500 – 1500)
•        Modern history (1500 – present)     
  o        Early Modern Period (1500 – 1750)     
  o        Mid Modern Period (1750 – 1914)   
   o        Contemporary Period (1914 – present)