In what way have mass extinctions catalyzed evolutionary radiation? Mass extinction events cause mutations, increasing the genetic variability of populations. Mass extinction events permanently alter environments, favoring species with novel character combinations. Mass extinctions wipe out poorly competing species, enabling good competitors to diversify. Mass extinctions eliminate ecologically dominant species, enabling survivors to diversify in environments with few competitors.

Mass extinction events permanently alter environments, favoring species with novel character combinations.

Explanation:

This is the most true and accurate answer but lets address why its the best choice.

A) "Mass extinction events cause mutations, increasing the genetic variability of populations" - Mass extinction events are not directly responsible for mutation in a population. Mutation is directly a result of issues during DNA replication / other long term exposures of certain mutagens (as seen in humans with UV radiation). Also mass extinction often limit genetic variability by wiping out large segments of populations.

C) "Mass extinctions wipe out poorly competing species, enabling good competitors to diversify" - This may not always be true. Dinosaurs were the predominant terrestrial animal prior to their extinction (the were not poorly competing species)

D) "Mass extinctions eliminate ecologically dominant species, enabling survivors to diversify in environments with few competitors." - Once again this is not entirely true, the issue with mass extinctions is that they in most cases entirely random events that do not discriminate against groups of populations because they are not easily survivable by a wide range of species hence the term mass extinction.

Now lets look at why B must be right.

"Mass extinction events permanently alter environments, favoring species with novel character combinations." - Mass extinction events such as major geological changes (volcanic eruptions) often change the environment as a result of their occurrence. In addition to changing environments they also wipe out a multitude of species and open up many new ecological niches that need to be filled. In the case of dinosaurs it is argued that after their extinction mammals filled their ecological niche as the dominant terrestrial animals. This is generally what is called adaptive radiation where these new species and genetic recombinants produced by natural selection in direct response to a environmental change are favored. This process is most readily seen after mass extinctions which is why, in my opinion, this answer fits the best.

b. a lightning bug, plug of grass, and a worm