Nevertheless, I could say that we are still supposed to find something, at least, opposite to lowblood temperature. Yes, no choice leads us to correct answer but I think the technique of elimination works perfectly. All choices except regulation are more or less consistent with low temperature, i.e., limiting, diffusing etc. internal temperature, which is the direct opposite to what we are looking for.

Only regulating looks neutral, not necessarily means decreasing temperature.

The correct answer is D) regulating.

Breakdown of the Blank

The sentence explains that the term "cold-blooded" does not mean an animal has constantly low-temperature blood. Instead, it means they are unable to do something with their internal temperatures.
- Regulating means to control or maintain the rate or speed of a process so that it operates correctly. This is the precise scientific term for an organism's ability to maintain its internal temperature, a process also known as thermoregulation. The ability to regulate their internal temperature is what separates warm-blooded animals from cold-blooded ones.