What is a key factor that defines the effectiveness of a safety fuse?

The effectiveness of a safety fuse is primarily determined by the consistency and rate of the burning medium within the fuse. This aspect is crucial because it directly influences the timing of ignition and the reliability of the fuse to function as intended. A consistent and appropriately rated burning medium ensures that the fuse ignites at a predictable rate, providing sufficient time for operators and spectators to maintain a safe distance from the fireworks before detonation.

Variations in the burning medium's characteristics can lead to unpredictable behaviors in ignition timing, which may compromise safety. Therefore, the formulation and design of the burning medium play a vital role in ensuring that the safety fuse performs effectively in various conditions, making it the defining factor of its overall efficacy.

While the other options may have some relevance, they do not have the same direct impact on the fundamental safety and reliability of the fuse as the characteristics of the burning medium.