The following are among the important barriers that commonly prevent other entering monopoly market except_____________a.economies of scaleb.control over ownership of an essential resourcesc.Illegal barriord.legal barriers

The question is asking which of the following is NOT commonly a barrier that prevents others from entering a monopoly market. The options are:

a. Economies of scale b. Control over ownership of an essential resource c. Illegal barrier d. Legal barriers

Let's go through each option:

a. Economies of scale: This is a barrier to entry. In many industries, as the volume of production increases, the cost of producing each individual unit declines. If a company is already established and producing at a high volume, new entrants, producing at a smaller scale, will find it difficult to compete.

b. Control over ownership of an essential resource: This is also a barrier to entry. If a company has exclusive control over a resource necessary for a particular industry, it can prevent others from entering the market.

c. Illegal barrier: This is NOT typically a barrier to entry. While illegal activities can occur in any industry, they are not a common or accepted method of preventing market entry.

d. Legal barriers: These are barriers to entry. They include things like patents, licenses, and regulations that can prevent new companies from entering a market.

So, the answer to the question is c. Illegal barrier.