The highest possible oxidation states of uranium and plutonium can be identified by considering the electronic configurations of these elements and noting the number of valence electrons available for bonding.

Uranium (U, atomic number 92) has the electronic configuration: $ [Rn] 5f^3 6d^1 7s^2 $

Plutonium (Pu, atomic number 94) has the electronic configuration: $ [Rn] 5f^6 7s^2 $

To find the highest oxidation state, we sum the number of $d$, $f$, and $s$ electrons in the valence shell.

For uranium :

$ 3 (f) + 1 (d) + 2 (s) = 6 $

So the highest oxidation state of uranium is +6.

For plutonium :

$ 6 (f) + 2 (s) = 8 $

So the highest oxidation state of plutonium is +8. However, in practice, plutonium is known to exhibit a highest oxidation state of +7.

Comparing with the given options, the correct answer is :

Option D : 6 and 7.