Which question was central to the marshmallow test?

The answer is "Would you be able to resist getting a small reward now in order to get a larger reward late?"

Walter Mischel who was a psychologist experimented in 20th century, around 1960-70, the  marshmallow experiment where a kid was given a choice between an instant small reward or a larger reward at a later point in case of not accepting the instant one. Usually that reward was a marshmallow.  

answer; i believe the correct answer is (crops);

for your question the answer would have to be france