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 entire national output produced and consumed in industrialized countries passes through commercial channels

the existence of a non-commercialized sector in different underdeveloped countries renders the national income comparisons over countries difficult

no part of national output should be produced and consumed without passing through commercial channels

a part of the national output being produced and consumed without passing through commercial channels is a sign of underdevelopment