Which of the following is an example of an organ in animals?

The heart is classified as an organ because it is a distinct structure composed of various types of tissues working together to perform specific functions essential for sustaining life. In the case of the heart, its primary function is to pump blood throughout the body, which is vital for delivering oxygen and nutrients to cells while also removing waste products.

Unlike blood, which is a fluid tissue that functions as a transport medium, or muscles and neurons, which are types of tissues that perform specific functions (contraction and transmission of signals, respectively), the heart acts as a structural unit that integrates these tissues to achieve a greater purpose. Thus, the heart's complexity and collaboration of different tissue types make it a prime example of an organ in animals.