Selective leaching can make an alloy:

Selective leaching is a process that occurs when a specific element within an alloy is preferentially removed, often due to electrochemical reactions with the environment. This can lead to a significant change in the alloy's properties.

When selective leaching happens, the removal of certain elements—such as zinc from brass or aluminum from aluminum alloys—can create a situation where the remaining material becomes weaker and more brittle. As the alloy loses its ductility and structural integrity, it can also develop porosity due to the absence of the leached-out material. This reduction in strength and increase in brittleness are critical aspects of why the alloy could be described as weaker, brittle, and porous following the selective leaching process.

In contrast, while selectivity leaching might have some benefits in specific contexts, such as creating a corrosion-resistant layer or modifying surface properties, the predominant effect tends to lead to a deterioration in structural performance, particularly if the leaching is left unchecked. This distinction underscores why the correct answer revolves around the unfavorable impacts of selective leaching on an alloy's physical characteristics.