Hi Ramsree,

Here’s why B-C-D-A is the correct sequence:
Sentence B starts by introducing the idea that advertising effectiveness is best understood on a per capita (individual) basis, setting the context for the whole discussion.

Sentence C builds on this by explaining how to calculate overall advertising effectiveness, taking into account both the number of buyers and the cost of advertising. This moves from individual effectiveness to overall impact.

Sentence D then prompts you to consider how much of the brand the average person is currently buying, tying back to the individual perspective introduced in Sentence B.

Sentence A concludes by suggesting the potential increase in sales through advertising, applying the previous ideas toward potential growth.

This sequence B-C-D-A effectively guides from understanding individual impact to considering overall effectiveness, current buying levels, and then how advertising could boost sales.

I hope this helps.