Which plane divides the body down the center into equal left and right portions?

The midsagittal plane is the correct answer because it specifically divides the body into equal left and right halves. This anatomical plane runs vertically down through the midline of the body, ensuring that both sides are symmetrical and of equal proportion. The term "midsagittal" emphasizes that it cuts through the center, as opposed to just any vertical division.

In contrast, the sagittal plane refers to any vertical plane that divides the body into left and right parts, but not necessarily of equal size. The parasagittal plane is similar to the sagittal plane but occurs off-center, creating unequal left and right sections. The horizontal plane, also known as the transverse plane, divides the body into upper (superior) and lower (inferior) parts, which is a different type of division altogether. Thus, the midsagittal plane is the precise plane that delineates equal left and right portions of the body.