During the late 1800s, an open immigration policy was most strongly supported by

Industrialists

Explanation:

Together with entrepreneurs preferred a comparatively open and flexible immigration policy – one "...which would safeguard a stable source of workers. Encompassed within this group were the chief steamship..." and railway businesses which seen immigrants as possible laborers and also vital passengers on their appearances and the basis of future traffic in terms of things and materials. (CREDIT: Some info found in History Textbook)