Which of these factors had a direct influence on the spread of postclassical empires?
a. expansion of transcontinental trade routes
b. the unification of europe under the roman empire
c. european exploration of the western hemisphere
d. the establishment of most major monotheistic religions

The best and most correct answer among the choices provided by your question is the first choice or letter A. The post-classical period follows the decline of the great empires. Areas under previous control of the Empires were now economically in decline. Among the main factors that influences the spread of post classical empires is "Expansion of transcontinental trade routes."

I'll go with that