The expansion of grasses during the oligocene and miocene epochs led to

a. the extinction of mammals.
b. the expansion of tree-dwelling primates.
c. an increase in dinosaur and reptile species on land.
d. the radiation of herbivores such as grazing mammals.

The answer is:

D. the radiation of herbivores such as grazing animals

Explanation:

Oligocene and Miocene Epochs:

The Oligocence and Miocene epoch are both attributed to the expansion of grasslands and savannah. Both eras marked rapid and drastic evolutionary changes in grazing mammals and herbivores. Diverse groups of grazing mammals lived throughout these eras. For example, the largest herbivore and land mammal of all time, Indricotherium (a sort of giant hornless rhinoceros), was present in the Oligocene era.

Similarly, the Oligocene era in North America favored the rapid radiation of primitive horses.

it is either c or d and i know for sure it is not a or b

i think that is new experiments yield untrustworthy data and should be carefully considered, because you don't know what will happen with the experiments.