Explanation:

- Option 1: Private sector firms

- Early phases of reform in China did focus on opening up to private business, but the later phase was more about other sectors.

- Option 2: Trade policy

- Trade reforms were important, but structural banking reforms came even later.

- Option 3: Investment sector

- Investment reforms have been ongoing, but not the major highlight of the later phase.

- Option 4: Banking sector

- In the later phase of reforms, China introduced significant reforms in the banking sector. This included restructuring state-owned banks, allowing foreign banks more access, and improving overall financial stability.