When was a state obligated to follow the constitution?
a. after all the states had ratified it
b. after that state had ratified it
c. after a majority of states had ratified it
d. as soon as it was written

I believe the answer is: B. after that state had ratified it
Ratification refers to the establishment of formal consent to follow a certain regulation.
After the states finished the ratification process, they would legally binded to every regulation that exist in the constitution and would received some sort of punishment if they failed to do so.