Here we must find which element has the highest electronegativity for that in a group, electronegativity decreases as the size increases, leading to its ability to attract electrons decrease. Thus, P<N and Si<C.  In a period, as the size decreases, electronegativity increases due to the increase in effective nuclear charge.  Thus, C<N and P>Si. 

Hence, the overall order is Si<P<C<N Therefore, N has highest electronegative element.