The correct answer is D. An ultrasound
Step-by-step explanation:
Blood circulation refers to the process in which blood moves to and from the cells of the body transporting nutrients, oxygen and other elements to all the body that are necessary for life and other processes such as regulating temperature. Additionally, this biological process implies the circulation of blood and therefore to observe this, doctors and other experts in the field require observation tools that provide real-time images. Due to this, from the options, the more suitable is an ultrasound because this uses sound waves to create images in "real-time" and thus, they can be used to study organs and also movement including circulation of blood.