Question

65 million years ago there was a mass extinction that is best known for killing the dinosaurs. there was also a large deposit of dust that contained substances only found in asteroids. what most likely killed off the dinosaurs?
an asteroid impact caused the sun to be blocked.
they were crushed by an asteroid.
the water supplies were polluted.
mammals evolved to be better competitors.

Explanation:

The presence of asteroid - specific substances in the dust deposit suggests an asteroid impact. An asteroid impact would kick up a large amount of dust into the atmosphere. This dust would block the Sun. Blocking the Sun would lead to a significant drop in temperature (a "nuclear winter" - like effect), disrupt plant growth (as photosynthesis requires sunlight), and ultimately cause the extinction of dinosaurs (as they would face food shortages, harsh climate changes etc.).
Crushing by an asteroid (if it was a direct hit on a large number of dinosaurs) is less likely as the extinction was widespread across different dinosaur species in various regions. Pollution of water supplies is not indicated by the asteroid - related dust evidence. Mammals evolving to be better competitors would be a slower process and not likely to cause a sudden mass extinction like the one 65 million years ago.

Answer:

An asteroid impact caused the Sun to be blocked.