Answer:
a. 50 thousand
Step-by-step explanation:
From the answer option, the closest true number is around 50 thousand.
It is estimated there are around 62 to 69 thousand species of vertebrates in the world.
Vertebrates are one of the major groups of animals and they are the ones that have the backbone. In the group of vertebrates, we can divide animals into further subgroups of amphibians, mammals, reptiles, fish, and birds. Therefore, we can see there is a large number of animals that we know of which are included in the vertebrates group.