Final answer:
Ohm's law states that the current is directly proportional to the voltage applied to substances that are ohmic materials. Resistance, which impedes current, is inversely proportional to current. The student's question is answered correctly by option d, as voltage and current exhibit a linear relationship in ohmic materials.
Step-by-step explanation:
The fundamental relationship that characterizes the interplay between current and voltage in ordinary solid matter is described by Ohm's law. Ohm's law states that the current (I) that flows through most substances is directly proportional to the voltage (V) applied to it. This means as the voltage increases, the current increases in a linear fashion, provided that the material is ohmic. For non-ohmic materials, this relationship does not hold, and the relationship between voltage and current can be nonlinear.
Resistance (R) is another important factor in the relationship between current and voltage. Resistance is the electric property that impedes current, acting similarly to friction in mechanical systems. According to Ohm's law, resistance is inversely proportional to current. In other words, as resistance increases, the current decreases, given a constant voltage.
Therefore, the correct answer to the student's question is option d: Voltage and current exhibit a linear relationship in solids, for materials that follow Ohm's law, which are known as ohmic materials.