I cannot understand why answer is D ... can someone pls help me.​

It is quite simple.

Lets start from the markings above the topic "external temperature."

3 lines after 20 C is 23 C. As you can see, both of them (animal 1 and animal 2 wave drawing on the graph) meet at that point. So thats why Option D is correct

what subject is this

evidence and logical support is essential for a scientist to do in order to maintain credibility. this is because no one can believe without any logic and without any evidence.hence a scientist can maintain credibility by proving himself and by answering logically so experiments prove very beneficial. logical hypothesis give conclusion which is acceptable. therefore, there are two main factors to maintain credibility by a scientist is evidence and logical support.