ANS:A - low thermal conductivity.

An insulator should have low thermal conductivity. Thermal conductivity is a measure of a material's ability to conduct heat. In the context of insulation, materials with low thermal conductivity are preferred because they inhibit the flow of heat, helping to maintain desired temperatures and reduce energy losses. Insulators are designed to minimize heat transfer, and materials with low thermal conductivity are effective in achieving this goal by resisting the flow of heat through them. So, the correct answer is: low thermal conductivity.