When is the energy of electron regarded as zero.

Question

$\begin{array}{1 1} \text{(a) when it is at infinite distance from the nucleus.} \\ \text{ (b) when it is at finite distance from the nucleus.} \\ \text{ (c) when it is at infinite distance from the protons.} \\ \text{ (d) when it is at infinite distance from the electrons.} \end{array}$