The answer that BEST describes why guest workers can be a problem for many countries is:
D: Guest workers are a problem for many countries because they frequently stay longer than initially permitted under the terms of their visa, thereby adding to the ranks of undocumented immigrants in their host country.
This statement highlights a significant issue related to guest workers, as overstaying visas can lead to legal and social complications in host countries, often straining resources and creating tension within the community.