Final answer:
One approach to reducing the inefficiency of marketing research in multiple countries is to group prospective markets into clusters. The most suitable grouping option is to group countries into clusters of homogeneous countries.
Step-by-step explanation:
When conducting marketing research in multiple countries, it can be inefficient to study each one individually. Therefore, one approach to reduce this problem is to group prospective markets into clusters. The most suitable option for narrowing down the list of countries to be examined is to group them into clusters of homogeneous countries. By categorizing countries that have similar characteristics and consumer behaviors, companies can gain insights into the overall market trends and preferences without having to study each country individually.