Four identical charges are placed at the four vertices of a square lying in YZ plane with origin at the centre of a
square. A fifth charge is moved along X axis. The variation of potential energy (U) along X axis is correctly
represented by
At the centre of the square net force on the charge is zero. However, in this position the charge is in unstable equilibrium and hence its potential energy is maximum and finite. The four charges are assumed to be placed symmetrically about the origin in YZ plane.