Maybe im stating the obvious here but if this is in reference to an IR skills test then the flight should be treated as a public transport flight(certainly used to be when I did mine, albeit some time ago now). If its still the case then I would be applying the relevant EU Ops criteria to select an appropriate alternate(if required....or even 2 alternates if legally required). Thats perhaps what your examiner was looking to see in your planning and may explain why he said your choice of alternate was wrong.