Final answer:
The most prevalent concern associated with underground storage tanks is leakage, due to its potential for environmental harm and health risks.
Step-by-step explanation:
One of the most prevalent concerns of underground storage tanks is leakage. This problem is significant because leakage can lead to contamination of soil and groundwater, potentially causing environmental damage and posing risks to human health. Underground storage tanks, used frequently for the storage of fuels, can corrode over time or be subject to structural failure, leading to leaks.
Regarding other options mentioned: while combustion, explosion, and fire can occur with stored substances, these are less likely and usually associated with specific conditions, such as a breach in the tank integrity coupled with an ignition source for fire or explosion. Leakage remains the most widespread issue due to its potential for slow, undetected environmental damage.