Final answer:
The best method for studying the metabolic rate in animals in the wild is the doubly labeled water technique, as it is accurate and non-intrusive for wild animals, contrary to other methods which may not be practical in field conditions.
Step-by-step explanation:
When studying the metabolic rate in animals in the wild, the best method is b. Doubly labeled water. This technique involves introducing water with isotopes of hydrogen and oxygen into the animal's system and then measuring their concentration in the animal's body after a specific period. As the animal metabolizes the water, the isotopes are lost at a known rate, which correlates with energy expenditure. This method is less intrusive and more accurate for wild animals than alternatives such as respirometry and indirect calorimetry which require more controlled environments or are less practical in the field. Direct calorimetry, on the other hand, is usually impractical due to the requirement for a controlled environment and elaborate setup.