Step 1: Sort the public domain into three parts.
State: governed by law and force. Market: governed by buying and selling. The remaining part is where people act by free choice for shared causes.
Step 2: Look for the clue words.
The words non-state, non-market and voluntarily point to one idea, which is the sphere of voluntary groups and associations.
Step 3: Match it with the options.
Society is too broad. Social organization and social institution describe how social life is arranged, not a sphere of voluntary action.
Step 4: Choose.
Civil society is the term for this voluntary sphere, which is option 3.
Step 5: Why the other options fail.
Option 1 says: Society. This does not fit the idea we settled on, so it cannot be the answer.
Option 2 says: Social Organization. This does not fit the idea we settled on, so it cannot be the answer.
Option 4 says: Social Institution. This does not fit the idea we settled on, so it cannot be the answer.
Step 6: Quick check.
Read the chosen option again in full: Civil Society. It agrees with the facts above, and each of the other three options clashes with them. So the answer stays the same after this second look.
Final Answer:
The correct term is civil society.
\[ \boxed{\text{Option 3}} \]