We have to find an assumption that is sufficient in itself and logical to fill the gap for the author's conclusion.
A. Citizens of Country 777 use the Internet frequently.
Not an assumption, so what? It doesn't fill the gap why the govt should encourage the introduction of competing companies.
B. The government of 777 is more focused on health problems than on Internet infrastructures.
Not an assumption, totally irrelevant, the argument's sole focus is on internet, not a comparison.
C. Internet infrastructures are complex and expensive to install.
Not an assumption, it rather weakens the argument because it explains why that one company charges high amount for internet.
D. A competitive market encourages companies to reduce prices.
Assumption, we are given that customers pay high amount for internet services and that is because there is only one company right now, if more companies come in and competition puts downward pressure on prices then it fills the logical gap and the author concludes that the government should encourage the introduction of competing companies.
E. There is only one Internet company in 777 because the others were unsuccessful and had to cease operations.
Not an assumption, it just explains why there is an existence of one company.