We hope that the government's new policies will ______ a period of economic growth.

"Usher in" means to mark the beginning of something; it fits perfectly in this context, as the people are hoping that the new government will bring economic growth; "set in" is also used to mark beginnings, but it is primarily used in a negative context, which is not the tone here. Hence, usher in is a better choice here. 

"Turn up" means to increase, and "set forth " means to present ideas, neither of which aligns with the semantic demands of the context.

Hence, the correct option here is (A) usher in