Answer: D. Transition metals — like all metals — form a giant metallic structure: a lattice of positive ions held together by a sea of delocalised electrons. Statement D describes this correctly.
A is wrong on mechanism: variable oxidation states arise because transition metals can lose different numbers of electrons from their 3d and 4s subshells (which are close in energy). They don't lose electrons into the metallic sea — that sea is what holds the lattice together in the first place.
B is wrong: although many transition-metal compounds are coloured, not all are blue or green. Manganate(VII) is purple, dichromate is orange, \(\text{Fe}^{3+}\) gives yellow-brown solutions.
C is wrong: manganese in \(\text{MnO}_4^-\) is in oxidation state +7 — higher than +6. Chromium in \(\text{CrO}_4^{2-}\) is +6 but +7 in Mn(VII) shows transition metals can exceed +6.
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