When faced with a sudden drop in environmental temperature, an endothermic animal will:
experience a drop in its body temperature
wait to see if it goes lower
increase muscle activity to generate heat
add fur or fat to increase insulation

add fur or fat to increase insulation

Explanation:

The endothermic animals are the animals that have developed an internal heating mechanism. This basically means that these animals do not depend on their environment for heat, but instead they produce it themselves. In order to be able to better produce their energy and keep it, these animals have developed traits that help them to increase the insulation, such as gaining on fat, fur, or feathers. The mammals are a prime example of endothermic animals, which is why they are successful in any type of environment.

the answer will be a) proteins i believe