If a material conducts heat easily, it is a good

Your answer would be conductor.

It would be thermal conductor

Explanation:

Thermal conductivity is attributed to the exchange of energy between adjacent molecules and electrons in the conducting medium. A substance of large thermal conductivity k is a good heat conductor, whereas one with small thermal conductivity is a poor heat conductor or good thermal insulator.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.