The correct answer is C) A gray-green patina formation. This is the correct answer because a gray-green patina formation is a characteristic indication of active corrosion on copper alloys. The gray-green coloration is caused by the formation of copper-based compounds, such as copper carbonate or copper chloride, which develop on the surface of the metal due to oxidation and environmental factors. The other answer choices are incorrect because: A) A white-gray powder formation is more indicative of silver corrosion, not copper alloys; B) A copper-oxide crust formation is not a typical sign of active corrosion, but rather a more advanced stage of oxidation; and D) A verdigris formation, which is a blue-green or blue-gray patina, is also a sign of copper corrosion, but it is distinct from the gray-green patina mentioned in the correct answer.