Respuesta :
The correct answer is A. For
Explanation
According to the above, the keyword that helps readers to understand what caused immigrants to be detained at the Ellis Island Hospital Complex Medical is the word "for" because in the paragraph the first phrase says "officers screened the immigrants for any signs of physical or mental illness". In this phrase, the author first announced that the officers had to examine the inmigrants, and later the objective of that exam to know if they were ill to be detained at the Ellis IslandHospital Complex. So according to the above, the correct answer is A. For.