Final answer:
A midsagittal section passes through the (Option b) heart, as it is the central structure that can be bisected into equal halves by such a plane. Other options like the spleen, lungs, kidneys, and legs would not be divided equally by a midsagittal section.
Step-by-step explanation:
A midsagittal section is a plane that divides an animal into equal right and left portions. In terms of human anatomy, this section would pass through midline structures. Therefore, it would pass through the heart as it is centrally located and is a structure that can be bisected into equal halves by a midsagittal plane.
Other options such as the spleen and the left lung would not be bisected by a midsagittal section as these organs are lateral and would be present on only one side of such a section. Similarly, organs like the kidneys and legs are paired structures, situated bilaterally, and a midsagittal section would only pass through one or the other, not bisecting either organ symmetrically. Hence the correct answer to the question is the heart, aligning with the definition of a midsagittal section.