Answer:
Silver, to prevent heat loss by radiation
Explanation:
The three methods of heat transfer are conduction, convection and radiation
A thermos tries to minimise all three.
The gap between the inner and outer materials is a partial vacuum which reduces heat transfer by conduction and convection.
The purpose of making the lining silver is that silver is the best reflector of heat so it keeps it inside the thermos rather than absorbing it. At the opposite end of the spectrum is black (the best absorber and emitter of heat) - a thermos with this colour on the inside would be much less effective!
