Which of the following oxide is most acidic?

The most acidic oxide is di-metal pentoxide, which is more acidic than metal dioxide and metal tetra oxide. This is because an element's oxides in higher oxidation states are more acidic than those in lower oxidation states. $M_2{\mathrm{O}}_5$ is the most acidic oxide as a result.

As oxidation number of central atom increases, acidic nature of oxide increases.

Hence, option D is correct.