Not sure between 2 answers on the question below.

What is the BEST way to write this sentence correctly?
"David went to the store and bought some bread then he took the long way home."

a. David went to the store and bought some bread. Then he took the long way home.
b. David went to the store and bought some bread, then he took the long way home.
Thanks..

B is incorrect. It's a run-on.

A is already correct; B would be correct with a semicolon in place of the comma.