Final answer:
The main challenge in preventing water from going bad in an electric water heater is the presence of bacteria in the water supply, which can proliferate in warm temperatures.
Step-by-step explanation:
The primary challenge faced by the user in preventing the water from going bad in a 200-liter electric water heater is D) Presence of bacteria in the water supply. The storage of warm water can encourage the growth of bacteria, so maintaining the right temperature to prevent bacterial growth is crucial.
Challenges like A) Temperature fluctuations during usage, and C) Limited insulation of the water heater, while important, primarily affect energy efficiency rather than water quality. B) Unpredictable visitation schedule and E) Inability to control water temperature remotely impact the user's convenience and control but are not directly related to water quality.