What characteristic of a flame indicates the permissibility of an explosive?

The characteristic of a flame that indicates the permissibility of an explosive is a short flame of short duration. This type of flame suggests that the explosive is capable of burning quickly and does not produce a prolonged or high-intensity flame, which can indicate a greater risk of hazardous conditions. Permissibility in this context relates to the safe use of explosive materials in mining operations and the importance of minimizing the potential for dangerous explosions or fires.

Explosives that produce a short flame are often regarded as being safer for use in underground mining, as they are less likely to create an environment conducive to secondary explosions or fires that can pose significant risks to personnel and equipment. In contrast, a long flame of high duration could signal a more powerful explosive reaction that might not be suitable for mines, where controlling explosions is critical for safety. Similarly, the other options that describe consistent or medium-length flames do not align with the characteristics required for permissibility, as they may indicate less controlled or potentially more dangerous explosive reactions.