The correct answer is Thorium.

Let`s dissect the options:

1) Plutonium: It`s not correct because plutonium is a by-product of nuclear reactions, not commonly found in building materials.

2) Thorium: It`s the correct answer. Thorium is a naturally-occurring radioactive material. Certain rocks, including some used in building materials, contain trace amounts of thorium. This is the reason thorium has been gaining public attention recently.

3) Radon: While radon is indeed a radioactive pollutant, it`s typically not present in building materials themselves. Rather, it can seep into homes from the surrounding soil and rock, especially in certain geographical areas.

4) Radium: This is also an incorrect answer. While radium is radioactive, its presence in building materials is not a significant concern in most cases.

So, Thorium is the best answer because it`s found in trace amounts in some building materials, which has raised concerns about its potential health impacts.