In order to reduce bias and increase reliability, what is the best way to code open-ended questions?

The persian wars started when emperor darius i began an invasion of greece. what caused darius to decide to attack greece? a. greek pirates had raided persia's merchant ships and coastal towns. b. greek armies had attacked and taken over land that belonged to persia. c. greek city-states had supported rebellions in the persian empire. d. greek leaders had declared independence from the persian empire.