Not genius but still wanna post my comments.
A. Althougth it sells all the telephones that it produces,Mammoth Industries' share of all telephone sales has declined over the last year.
Mammoth has a good performance for selling out all its telephones but there is no information about whether the sale is decreasing or increasing. Although the share of telephone sales declined, it cannot be concluded that the sale is declining. Anyway, the share is a ratio with a denominator of Mammoth's total production including other products.
Additionally, it is not saying the sales would increase this year if it increased last year.
B. It is mentioned in question that sales of telephone increased last year. Note that it is not necessarily referred to as the sales of Mammoth's telephones. In fact, Mammoth's sales of telephone might have either decreased or increased. The decrease of average inventory may due to reducing production or increasing sales. Your conclusion is inappopriate.
C. Whether the customers know it is Mammoth who owns the brand of certain types of telephone has no relation to the sales, anyway.
D. Though it says sales increased last year. No evidence is given for the definite applicability of the plan proposed for this year.
Actually, A, B and D did mention something related to sales of last year but are not supportive for the increase of sales of this year. C is irrelevant information.
E. It is mentioned in the question that Mammoth's advertisement has already been developed intensively last year. Choice E reveals another attempt of Mammoth to stimulate the sale of telephones last year, which is to reduce the price. However, it is implied that both advertisement and price reduction have failed for the fact that the sales fell last year given in Choice E. It proves the prediction that the plan is not applicable. | 
