Why does lymph need to flow more slowly through the lymph nodes?
1) so that lymph can produce more lymphocytes
2)so that lymph does not backflow into surrounding organs
3) so that lymphocytes can destroy harmful substances in the lymph
4)so that lymph can diffuse and dilute the harmful substances

The right answer is 3).

Lymph nodes are the place of proliferation and differentiation of immune cells. The primary role of the lymphatic vessels is to bring an antigen (in the vast majority of cases from a pathogen) from the tissue to the ganglia, to activate the specific immune response of T and B cells.

The fact that the circulation is slow favors the contact between the lymphocytes at the level of the ganglia and the antigens.

C. so that lymphocytes can destroy harmful substances in the lymph

(on e2020) xD