Which of the following increases the conductivity of a substance?

The correct choice regarding what increases the conductivity of a substance is higher temperature. As temperature rises, the kinetic energy of the particles within the substance increases. This higher energy allows ions and electrons to move more freely and rapidly, thereby enhancing the material's ability to conduct electricity.

In many conductive materials, especially electrolytes, an increase in temperature can lead to a higher degree of ionization, which directly contributes to improved conductivity. This relationship is especially evident in ionic solutions, where heating can increase the concentration of mobile ions available to conduct current.

Other options such as less moisture, fewer ions, and thicker materials would either decrease conductivity or not have a positive impact. Less moisture typically means less available charge carriers (like ions), fewer ions directly reduces the number of charge carriers that can carry current, and thicker materials may offer more resistance to the flow of current. Thus, these factors do not contribute to increasing conductivity in the same way that higher temperatures do.