The correct answer is C. remains constant.
In constant-current charging, the charging current is kept constant until the battery reaches a certain voltage, called the cut-off voltage. At this point, the charging current is reduced to a trickle charge to maintain the battery’s charge.
The supply voltage in a constant-current charging method remains constant throughout the charging process. This is because the charging current is kept constant, and the voltage across a resistor is proportional to the current flowing through it.
Option A is incorrect because the supply voltage does not decrease throughout the charging process. Option B is incorrect because the supply voltage does not increase throughout the charging process. Option D is incorrect because the supply voltage does not change throughout the charging process.