Answer:
4. The voltage is reduced by a factor of 2
Step-by-step explanation:
The relationship between voltage, current and resistance in a circuit is given by Ohm's law:
V = RI
where
V is the voltage
R is the resistance
I is the current
In this problem,
the resistance in the circuit is halved: R' = R/2
the current is constant : I' = I
So the new voltage is

so the voltage is reduced by a factor 2.