Let me help you guys. Here is the step by step process.
1. What is companies position.
Yet, the company's management claims that the campaign had no impact on its sales. ----- in simple words this campaign has nothing to do with this.
2. pre-think - Think on the lines, why company is adamant that campaign has nothing to do with this decline. here are my thoughts.
- The decline started just before the campaign started.
- everyone is facing decline, it means something happened in general and nothing to do with campaign .
A is on these line, so A must be the right answer. But as you have asked, let see why E can not be the answer.
E. Most customers of soft drinks are loyal to their preferred brand and hardly ever replace it with other brands of the same category.
---- We need to establish that why campaign has nothing to do with it. Now for this statement and this choice to be true, we need to make many assumptions. such as what about new people. It may possible for campaign to attract new people(Those who don't drink any of it.) campaign could have attracted them. also existing customer should not leave too. after all its loyalty, but decline means some customer left as well.
Hope it is helpful.