{c}

Every vertex has degree three, so its incident edges require three distinct colours. Three suffice by colouring an edge according to which of the three coordinates changes; every vertex has exactly one edge of each coordinate type. Thus \b[the <edge chromatic number> is three].

Solution

Past exam of the probability, statistics and options I course of the University of Oxford / 2009 / R / 1 / ii / b / Solution

 Solution (source code)