The energy loss of electrons in water or tissue is roughly given by:

The energy loss of electrons traveling through water or tissue is primarily described by the Bethe-Bloch formula, which states that the energy loss per unit distance varies with the electron's energy. For electrons, this energy loss reaches a relatively consistent average value in the region of a few MeV.

In tissue or water, the average energy loss is approximately 2 MeV/cm. This value reflects the balance between the energy lost to ionization and excitation of electrons in the material. As electrons pass through biological tissue or water, they interact with the atoms in the medium, losing kinetic energy mainly through ionization processes, which is effectively quantified around this magnitude.

As electrons get lower in energy, their rate of energy loss becomes more pronounced in terms of MeV per centimeter. The choice of 2 MeV/cm as a general approximation for energy loss in tissue is commonly accepted within the field of medical dosimetry and radiation therapy, which helps in planning and delivering accurate treatment doses. Hence, the option listed as 2 MeV/cm is consistent with established dosimetric principles and empirical findings in radiation physics.