Answer:
B. Today's mammals include numerous, diverse species that fill a wide variety of niches.
Step-by-step explanation:
The adaptive radiation is a type of evolutionary mechanism that can be represented by an accelerated process of diversification. This evolutionary process is produced when a taxonomic group is able to exploit new niches, when they have more availability to obtain food, etc.
In the case of mammal evolution, the extinction of dinosaurs enabled this group to make use of a wide range of new niches, thereby leading to its diversification.