Explanation:

Gold mining activity is a source of mercury pollution in the world.

True. In many small-scale gold mining operations, especially artisanal mining, mercury is used to separate gold from its ore. This process leads to mercury pollution in waterways and the atmosphere.

Coal-based thermal power plants cause mercury pollution.

True. Coal contains traces of mercury. When coal is burned in power plants, the mercury is released into the atmosphere, contributing to air pollution.

There is no known safe level of exposure to mercury.

True. Mercury is a potent neurotoxin, and exposure can lead to a range of health issues. Even low levels of exposure can be harmful over time, especially to vulnerable populations like pregnant women and children.

Given the above evaluations, all three statements are correct.