C. state-owned businesses.
A mixed-market economy is one where both private and public sector (government) involvement coexist. The establishment of state-owned businesses is a key sign of this type of economy, as the government plays a significant role in owning and controlling certain industries or sectors, while private businesses operate in other areas. In a mixed-market economy, the government and private sector balance control over the economy, which distinguishes it from purely market-driven or command economies.
Other options, like a fair labor market (A), anti-poverty regulations (B), and a smaller middle class (D), may exist in various economic systems but are not as direct a sign of transitioning to a mixed-market economy.