The conceptual difficulties in National Income comparisons between underdeveloped and industrialised countries are particularly serious because a part of the national output in various underdeveloped countries is produced without passing through the commercial channels. In the above statement, the author implies that:

The author's statement highlights a specific problem: "conceptual difficulties in National Income comparisons" between different types of countries. The reason given for this difficulty is that "a part of the national output in various underdeveloped countries is produced without passing through the commercial channels." Let's analyze each option: A) the entire national output produced and consumed in industrialized countries passes through commercial channels - The statement does not explicitly or implicitly claim that the *entire* output in industrialized countries passes through commercial channels. It only contrasts the situation in underdeveloped countries, where the non-commercialized sector is significant enough to cause comparison difficulties. This is an overstatement not directly supported. B) the existence of a non-commercialized sector in different underdeveloped countries renders the national income comparisons over countries difficult - This option directly rephrases the core implication of the original statement. The "non-commercialized sector" refers to the output produced without passing through commercial channels. The statement explicitly says this situation "renders the national income comparisons... particularly serious" (i.e., difficult). This perfectly matches the author's implication. C) no part of national output should be produced and consumed without passing through commercial channels - This is a prescriptive statement ("should be"). The author's statement is descriptive, identifying a factual difficulty in economic measurement and comparison, not making a value judgment or suggesting how economies ought to be structured. D) a part of the national output being produced and consumed without passing through commercial channels is a sign of underdevelopment - While a larger non-commercialized sector is often *associated* with underdevelopment, the author's statement focuses on the *difficulty it creates for national income comparisons*, not on it being a *sign* or definition of underdevelopment itself. The primary implication is about the measurement challenge, not a diagnostic criterion. Therefore, option B is the most accurate and direct implication of the author's statement.