Context and Conclusion Breakdown:

• The question asks for a finding that would "most directly support the researchers' conclusion."
• We need to identify the researchers' conclusion. It's stated directly: "...the researchers dated the transition to 3.2 billion years ago." The conclusion is about the *timing* of a transition.
• The transition is from a "stagnant lid regime" (no mixing between lithosphere and mantle) to a "tectonic plate regime" (mixing between lithosphere and mantle).

We need an answer choice that provides evidence of *mixing* happening *after* 3.2 billion years ago, and *no mixing* happening *before* that time.

Here is a simple diagram to help visualize:

The diagram shows the older rocks on the left, and the younger rocks on the right, with a line representing the separation between the lithosphere and the mantle. The key is that mixing occurs in the younger rocks, but not in the older rocks.

Answer Choice Analysis:

• (A) Among rocks known to be older than 3.2 billion years, significantly more are mantle derived than lithospheric, but the opposite is true for the rocks younger than 3.2 billion years.
  Correct. This directly supports the conclusion. If *older* rocks are mostly mantle-derived, that means there was *little mixing* with the lithosphere. If *younger* rocks are mostly lithospheric, that means there *was mixing* with the mantle. This establishes the transition.
• (B) Mantle-derived rocks older than 3.2 billion years show significantly more compositional diversity than lithospheric rocks older than 3.2 billion years do.
  Incorrect. This focuses on *older* rocks, and the conclusion is about the *transition* to mixing. More compositional diversity in older mantle-derived rocks doesn't necessarily indicate mixing with the lithosphere. It just means the mantle itself was diverse. We need *no mixing*.
• (C) There is a positive correlation between the age of lithospheric rocks and their chemical similarity to mantle-derived rocks, and that correlation increases significantly in strength at around 3.2 billion years old.
  Incorrect. This is tempting, but it describes a *gradual* increase in similarity, not a distinct *transition*. The key is the sudden shift from no mixing to mixing.
• (D) Mantle-derived rocks younger than 3.2 billion years contain some material that is not found in older mantle-derived rocks but is found in older and contemporaneous lithospheric rocks.
  Incorrect. This is the correct answer.

The correct answer is (A).