According to the quantum mechanical model of the atom, asking, "where is the electron? " is not nearly as as asking this question : a) where are the protons? b) where are the regions of electron density? c) where are the regions of proton density? d) where are the neutrons?

The correct answer is option B. Where are the regions of electron density?

In the quantum mechanical model of the atom the exact location of an electron is unknown. But there is a probability of finding an electron in a given space. So in quantum mechanics instead of an electron we always refer to electron density, which is simply the probability of finding an electron in space.