The trace on an oscilloscope continually moves to the right of the screen when:

The correct answer is option 2):(the sweep is  triggered)

Concept:

• The trace on an oscilloscope screen is a graph of voltage against time
• The nature of the graph is determined by the input signal of the oscilloscope
• The trace in a CRO is performed by sweeping electrons for a short period of time on the phosphorous screen by the voltage generated due to the signal. This voltage is known as sweep voltage.
• When the ‘sweep’ is completed the voltage returns to its original value and thus forcing the beam of electrons to move back. This process is called retrace.
• The trace on an oscilloscope continually moves to the right of the screen when the sweep triggered
• A triggered sweep can display such signals, and those of short duration, e.g. narrow pulses. In triggered mode, the input signal is used to generate substantial pulses that trigger the sweep. Thus ensuring that the sweep is always in step with the signal that drives it