The correct option is Length.
Explanation
The electrical resistance ($R$) of a uniform metallic conductor is governed by its physical dimensions and the intrinsic properties of the material. The relationship between resistance and the geometry of the conductor is fundamental to current electricity.
Detailed Analysis:
- Length: The resistance of a conductor is directly proportional to its length ($L$). As the length of the path for electron flow increases, the collisions between electrons and positive ions increase, thereby increasing resistance ($R \propto L$).
- Cross-Sectional Area: Resistance is inversely proportional to the area of cross-section ($A$). A wider path allows electrons to flow more easily ($R \propto 1/A$).
- Temperature: While the resistance of metals generally increases with temperature, the relationship is defined by a temperature coefficient ($\alpha$) and is not a simple direct proportionality in the same fundamental geometric sense as length.
- Mass and Velocity: Resistance is not directly defined by the mass or velocity of the conductor in standard circuit theory.
Combining the geometric factors, the formula for resistance is:
$$R = \rho \frac{L}{A}$$
Where $\rho$ (rho) is the electrical resistivity of the material.
Key Takeaway:
For a uniform metallic conductor, electrical resistance is directly proportional to its length and inversely proportional to its cross-sectional area.