Covalent bonds conduct electricity well.

always
usually
rarely
never

Covalent bonds never conduct electricity well.

 Option D

Explanation:

For a metal or conductor to conduct electricity, there must be free electrons moving freely on the outermost orbit so that when there is a potential difference, the electrons flows and produce electricity by theory motion.

Hence the outermost shell must be unstable for a conductor, else there will be no free electrons. In covalent bonds, electrons are shared which makes them difficult to move and produce electricity.

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask