Key question what are the goals of science?

One goal of science is to provide natural explanations for events in the natural world. Science also aims to use those explanations to understand patterns in nature and to make useful predictions about natural events. An organized way of gathering and analyzing evidence about the natural world.

Explanation:

Most likely to be A which is probably an artery which carries blood away from the heart Nd tens to carry oxygen rich blood

the leg doesnt contain aveoli this is found in the lungs so its not B
since its oxygen rich little carbon dioxide will be present in this structure so its not C
this part cant be part of the respiratory system as the respiratory system includes lungs, mouth and etc but not the legs so it cant be D

hope that helps