In a CRO (Cathode Ray Oscilloscope), the electron beam is swept horizontally across the screen at a steady rate so that the horizontal axis faithfully represents time. This is done by applying a sawtooth (ramp) voltage to the horizontal deflection plates. For the horizontal position to be directly proportional to time, the sweep voltage must rise linearly with time.

The way the timing capacitor is charged decides whether this ramp is linear:

• Charging through a resistor from a fixed supply gives V(t) = V_cc(1 − e^−t/RC). This is an exponential curve — it rises quickly at first and then flattens, so equal time intervals do not map to equal horizontal distances. This would distort the displayed waveform.
• Charging with a constant-current source keeps the current I fixed, so V(t) = (I/C)·t. Since I and C are constant, voltage increases at a uniform rate, producing a perfectly straight ramp.

Hence constant-current charging is preferred because the capacitor voltage rises uniformly over time, forming a linear sweep, giving an accurate time base.

The option stating the voltage rises exponentially and distorts the sweep actually describes resistor charging, which is exactly what constant-current charging avoids. Constant-current charging does not lengthen the retrace — retrace (flyback) is deliberately made short so it is nearly invisible. And the claim that linearity is set by the transistor collector current is misleading: the constant current only stays constant if the source is designed to hold it fixed regardless of capacitor voltage; linearity comes from the constancy of the current, not merely from using a transistor.