Answer:
B) All the beavers in the park.
Explanation:
He randomly picked 10 beavers out of all of the park out of all the beavers and weighed them. The 10 beavers wouldn't be all of the population otherwise he would say he took the 10 beavers that the park had not "A researcher wants to know the average weight that all the beavers in the park weigh. He randomly selects 10 beavers."