The CCC did many thing including (Choi’s more than one)
.building and maintaining park has infrastructure (bridges, roads)
.building public works projects like museums, school and community center
.planting millions of trees the country

It's all of them.

- building and maintaining park has infrastructure (bridges,roads)

- building public works projects like museums,school and community center

- planting millions of trees the country

Explanation:

CCC stands for The Civilian Conservation Corps. It's a program that gained the peak of its popularity in 1933 to 1942   that is designed to give unemployed people with jobs that related to public work.

They do various type of public works, including:

- building and maintaining park has infrastructure (bridges,roads)

They were involved in the construction of several park infrastructures. Most notably  The route of a parkway in Texas which they built in 1934.

-  building public works projects like museums,school and community center

The CCC has built more than 20 museums in total. Ranging from mountain caves museum to the conservation of wild life. They also involved in the building of local schools and community center that handles delinquents.

- planting millions of trees the country

The millions threshold is achieved within the period of their peak years alone. This movement actually still survive until today. Approximately tree planting movements which started by CCC has planted around 3 Billion Trees by the year of 2019.

the structure was most likely used for;

c. playing a ball game with stone hoops