The first statement in the sequence should be the one that introduces a new topic. It must not be a continuation or extension of a topic.
d is first in the sequence: d is the only one that can starts a sentence. It indicates that the subjects do something to save money.
d is followed by b: b continues by telling us what they do. It says that to save money the subjects usually buy.
b is followed by c: c tells us what they buy. it says that they usually buy flour and rice.
c is followed by a: a tells us in what manner they buy the flour and rice. It says that they buy the flour and rice for the whole year in one go.
Finally, the correct sentence is 'To save money we usually buy flour and rice for the whole year in one go'.
Thus, the correct sequence is d b c a. Hence, A is the correct option.