C

Step 1: Understand the function of an electric fuse wire.
A fuse wire is designed to melt and break the circuit when the current flowing through it exceeds a certain limit, thereby protecting the circuit from damage due to excess current.
Step 2: Analyze the properties of the fuse wire:

• Specific resistance: This is important because a wire with high resistance will heat up more for the same current, hence it will be more effective as a fuse.
• Radius of the wire: Thinner wires have higher resistance and will melt at lower currents, making this a critical parameter for the fuse.
• Length of the wire: The length also affects resistance. A longer wire has higher resistance and can affect the fuse's current rating.
• Current flowing through the wire: This is crucial as the primary function of the fuse is to interrupt the circuit when the current exceeds a safe limit.

Step 3: Determine which factor is immaterial: The length of the wire is important for determining resistance but does not directly influence the melting criteria for a given current and specific resistance. Therefore, its exact value is immaterial compared to the other factors.
Therefore, the correct answer is Option C: Length of the wire.