What is immaterial for an electric fuse wire

An electric fuse is a safety device that protects electrical circuits from excessive current.
The power supply is cut off if a large current flows through the fuse and causes it to burn. 
The thickness of the fuse helps in deciding how much current can be passed through the fuse wire before it melts.
The specific resistance of the fuse wire is important for its heating.
However, the length of the fuse wire is immaterial.

Therefore, the required option is "its length".