First Report: "Most Americans who consume monosodium glutamate regularly are not being harmed."
Second Report: "Most Americans who consume monosodium glutamate do not show visible symptoms of harm by the substance"
Pre-thinking: Consuming MSG caused harm but is not visible, or only visible in long-term
A. Some Americans who consume monosodium glutamate regularly are being harmed by the substance. - some Americans but we are talking about Most of Americans
B. Monosodium glutamate could be causing long term damage for which symptoms have not yet become visible. - Correct answer choice. If it is the case that MSG caused long term damage that currently not visible, so we need to change the statement that eating MSG does do some harm but not showing effect yet.
C. The report does not compare harm caused to Americans who consume monosodium glutamate with harm caused to people in other countries. - out of scope. only interested in Americans
D. Monosodium glutamate has been added to a growing number of commonly consumed foods during the past fifteen years. - out of scope
E. The severity of damage by monosodium glutamate differs from person to person. - out of scope
Therefore B is the correct answer choice