The question pertains to the output characteristics of a transistor, specifically focusing on the behavior of the collector current. In the typical output characteristics of a bipolar junction transistor (BJT), plotted as a graph of collector current (I_C) versus collector-emitter voltage (V_{CE}) for different base currents, the curves are analyzed to understand transistor operation.

Let's delve into each option to determine the correct answer:

1. Alternating continuously: This is incorrect because the nature of the output characteristic curves reflects the stable operation of the transistor under given conditions, rather than alternating behavior.
2. Zero for all voltages: This is incorrect. In the active region of transistor operation (where the horizontal curves are evident), the collector current is not zero. Instead, it takes a certain value determined by the base current.
3. Highly dependent on V_{CE}: This is incorrect for the region where the curves are nearly horizontal. The presence of horizontal curves suggests limited dependence on V_{CE}, as the collector current remains stable over a range of V_{CE} values.
4. Nearly constant for fixed base current: This is correct. In the active region of the transistor's output characteristics, the collector current is largely determined by the base current, remaining almost constant over a range of collector-emitter voltages when the base current is fixed. Hence, the horizontally-flat portion of the curves indicates that I_C remains nearly constant for a fixed base current.

Conclusion: The correct answer is Nearly constant for fixed base current as the horizontal curves in the output characteristics properly reflect this behavior. The output characteristic curves are a visual representation of the transistor operating in the active region where the collector current is controlled by the base current and remains steady across changes in V_{CE}.