Which of the following causes an increase in the resistance of a conductor?

An increase in the resistance of a conductor is influenced by the physical characteristics of the material. When the length of a conductor increases, the resistance also increases because the electrons that flow through the material will face more obstacles over a longer path. The longer the conductor, the more opportunities there are for the electrons to collide with the atoms of the material, which impedes their flow and results in higher resistance.

The decrease in diameter actually has the opposite effect, leading to higher resistance because a smaller cross-section restricts the flow of electrons, but this is more related to the geometric shape than the absolute length. An increase in temperature generally increases the resistance for most materials, particularly conductors, since higher temperatures can cause more vibrations in the atomic structure, impeding electron flow. A change in material will also affect resistance, as different materials have intrinsic properties that define their resistivity.

Thus, the most direct factor among the options provided that results in increased resistance is the increase in length of the conductor.