A sample of a pure element is a good conductor of electricity but is a poor conductor of heat (a thermal insulator). This element is most likely a:

The most likely answer is B) metalloid.

Explanation: Metalloids are elements that have properties intermediate between metals and nonmetals. They are typically good conductors of electricity due to their metallic luster and are often used in semiconductors. However, many metalloids are also poor conductors of heat, making them thermal insulators. This dual behavior makes metalloids unique compared to pure metals, which generally conduct both electricity and heat well, and nonmetals, which are usually poor conductors of both.