Premise: Almost all of the residents of Lonelitown drink cheap bottled mineral water, and not the water supplied by the municipality
Conclusion: Municipality should reduce the price paid for water if they are to encourage residents to go back to drinking it
Gaps: The author makes a jump by directly going to the price, there can be other possible factors as well like water quality, availability of water, etc.
A. The bottled water is imported from springs overseas.
No Impact, this option doesn't explain why the prices should be reduced so it can't be an assumption.
B. The municipality's water supply is filtered through a sophisticated process.
No Impact, maybe bottles also undergo highly sophisticated process, this option doesn't explain why the prices should be reduced so it can't be an assumption.
C. Residents of Lonelitown are not dedicated to the livelihood of their municipality.
No Impact, this option doesn't explain why the prices should be reduced so it can't be an assumption.
D. The price is the factor that most influences the resident's choice of drinking water.
Strengthener, if price were not the factor then why the author would have concluded the price perspective. The argument totally breaks down if this option was not true. This is the
assumption.
E. Bottled mineral water receives far more public attention through advertising than municipal water does.
No Impact, far more attention doesn't mean it is the reason for people's final choice. This option doesn't explain why the prices should be reduced so it can't be an assumption.