In the tempest, why does ariel do as prospero orders him?

a.
ariel thinks that prospero and miranda are going to leave the island soon, and he wants to convince them to stay.

b.
ariel wants to make prospero trust him so that he can eventually escape from his master.

c.
prospero has promised to set ariel free if the good spirit does what prospero says.

d.
prospero is very wealthy, and he pays ariel quite well for following orders.