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

The correct answer would be increase muscle activity to generate heat.

Explanation:

Endothermic animals are those which maintain their body temperature mainly by their metabolic activity irrespective of environmental heat.

In case, there is a drop in environmental temperature then endothermic animals increase their metabolic activity and metabolize sugars and fats at a faster rate.

It release more heat and increase the body temperature.

One the various methods for increasing metabolism is increase in muscle activity such as done in shivering. Shivering increases muscle activities which increases body temperature.