d
The oxide with the highest positive oxidation state on the element other than \(O\) should be most acidic. Oxidation state of \(V\) in \(V_2O_5\) and \(N\) in \(N_2O_5\) are \(+5\). But the electronegativity of \(N\) is higher, making \(N_2O_5\) the most acidic oxide.