Which of the following elements can affect the operational security limits in a power system?

The correct choice reflects that both the length and the impedance of transmission lines are significant factors in determining the operational security limits in a power system.

When considering transmission lines, length directly affects the resistance and reactance encountered in the circuit. Longer lines generally introduce higher resistance and may lead to increased losses, which in turn can necessitate more conservative operational limits to maintain system security and reliability.

Impedance plays an equally critical role, as it determines how much current can flow through the line under certain voltage levels. Higher impedance can limit the maximum power transfer capability of the line, thereby affecting operational limits based on the system's capacity to handle loads without risking instability or failure.

Operational security limits are influenced by the characteristics of the transmission system, and both the length and impedance are integral to these characteristics, ensuring that power flows remain within safe and reliable boundaries. Other factors, such as the number of transmission lines, while relevant, do not provide the same direct effect on operational security limits as length and impedance do, making the combination of both length and impedance the most comprehensive answer.