The image illustrates the concept of homotopic protons in organic chemistry. Homotopic protons are chemically and magnetically equivalent. They can be interchanged by a symmetry operation (like rotation about an axis of symmetry) of the molecule. If replacing each of two protons with a different group (e.g., deuterium) leads to identical compounds, then the original protons are homotopic. In the example of trans-1,2-Dichlorocyclopropane shown: The molecule possesses a C2 axis of symmetry*. This axis passes through the midpoint of the bond between the two chlorine-bearing carbons and the third carbon (C3). Rotation of the molecule by 180 degrees about this C2 axis* interchanges the two hydrogen atoms, H$^a$ and H$^b$, on the C3 carbon. Because H$^a$ and H$^b$ can be interchanged by a symmetry operation, they are considered homotopic*. Implication: Homotopic protons are indistinguishable and will give a single signal in Nuclear Magnetic Resonance (NMR) spectroscopy.