Which of the following measures can best check dictatorship in a state?

The correct answer is B. Separation of powers.

Explanation: The separation of powers divides government responsibilities into distinct branches (typically executive, legislative, and judicial), ensuring that no single entity has all the power. This system creates checks and balances, which can effectively limit the concentration of power associated with dictatorship. The other options, while important, do not directly address the structural balance needed to check dictatorial authority.