The passage says there is still sale of Arcadian Computers in Lyonnese even after imposition of Tariff to protect Lyonnese computer industry.
Lets analyze options for finding out reason best explaining the claim:
A Most computer retailers in Lyonesse have seen an increase of 40% in sales of Arcadian computers for this year.
Factual information and irrelevant. Eliminate.
B As retaliation against the new tariff, Arcadia has imposed tariffs on various products from Lyonesse.
Factual information and irrelevant. Eliminate.
C The computers from Arcadia are essential for the air traffic control systems in Lyonesse, and cannot be replaced by computers built locally.
This can be reason why there is still requirement of Arcadian computers even after imposition of tariffs. Keep this
D Before the imposition of the tariff, the cost of manufacturing a computer in Arcadia was comparable to the cost of manufacturing it in Lyonesse.
Again irrelevant information eliminate.
E Most experts agree that the computers built in Lyonesse are generally of a better quality than those built in Arcadia.
No explanation for the claim.
Correct answer is C.