Final answer:
In a uniform electric field between parallel horizontal conducting plates, a negative charge placed exactly midway between the two plates will remain stationary.
Step-by-step explanation:
In a uniform electric field between parallel horizontal conducting plates, a negative charge placed exactly midway between the two plates will remain stationary (option C). This is because the electric field between the plates is perpendicular to the plates and is constant in both magnitude and direction (Essential Knowledge 2.C.5). The negative charge experiences an equal and opposite force from both plates, causing it to stay in equilibrium at the midpoint. For example, if the negative charge tries to move towards the positively charged plate, it will feel a stronger repelling force that will push it back toward the negatively charged plate. Similarly, if it tries to move towards the negatively charged plate, it will feel a stronger repelling force that will push it back toward the positively charged plate.