To decrease the negative impacts from a potential eruption of Mount Rainier, the most effective option among those provided would involve measures that increase awareness and preparedness rather than trying to stop or ignore the situation. Therefore, the most reasonable response would be:
B. reduce population growth in areas that are in the pathway of lava.
This option can lead to fewer people being at risk if an eruption does occur. It is crucial for communities near active volcanoes to have emergency plans, warning systems, and education on volcanic hazards. Options A, C, and D do not effectively address safety or preparedness.