Final answer:
Geologists tested the asteroid impact hypothesis proposed by Walter and Luis Alvarez by measuring high levels of iridium in the K-Pg boundary's clay layer, examining the composition and fossil record of the clay, and through the eventual discovery of a matching-aged impact crater in the Yucatán Peninsula. Option number C is correct.
Step-by-step explanation:
In 1980, Walter and Luis Alvarez, along with their colleagues, proposed their asteroid impact hypothesis to explain the extinction event at the end of the Cretaceous period, also known as the K-Pg mass extinction. The hypothesis centered on a layer of iridium - an element rare on Earth but common in asteroids - found within a geological boundary known as the K-Pg boundary, which marks the end of the Cretaceous period and the beginning of the Paleogene period.
Geologists tested this hypothesis by examining various evidences. These included analyzing the composition of the clay layer where the iridium spike was discovered, which could suggest extraterrestrial origins. They measured the levels of iridium in the clay layer to confirm the abnormal spike. Furthermore, they studied the fossil record within the clay layer to understand the biodiversity collapse correlating with the timeline of the purported asteroid impact.
The final piece of evidence that bolstered the hypothesis was the discovery of a large impact crater in the Yucatán Peninsula, which matched the age of the iridium layer and was consistent with being caused by an asteroid of the magnitude necessary to disperse iridium at such a global scale and cause a mass extinction.