Answer: D. A metal's reactivity determines how easily it can be extracted from its ore. More reactive metals hold onto their oxide/compound state more tightly, so they are more difficult to reduce. Hence the most reactive metals (e.g., K, Na, Ca, Mg, Al) are the hardest to extract — they require electrolysis.
A is wrong: copper can also be extracted by reduction of its oxide with carbon — electrolysis is used for purification (refining), not initial extraction.
B is wrong: heating with less reactive metals can't displace more reactive metals from their compounds (only more reactive metals displace less reactive ones).
C is wrong: the least reactive metals can often be extracted by simple heating alone (e.g., HgO → Hg) or by reduction with carbon — hydrogen is not the standard reductant.
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