Question

An insulated lunch bag keeps food warm by

increasing the thermal energy of the lunch bag.
increasing the thermal energy of the food.
slowing the transfer of thermal energy from the air outside the bag to the food inside the bag.
slowing the transfer of thermal energy from the food inside the bag to the air outside the bag.

Answer 1

slowing the transfer of thermal energy from the food inside the bag to the air outside the bag.

Explanation: an insulated bag reduces heat flow (by conduction, convection and radiation), preventing the food's thermal energy from escaping. It doesn't add heat to the food.