The correct answer is: B. Decrease
An ammeter is a device used to measure the current in a circuit. It is connected in series with the circuit, which means that the current must flow through the ammeter in order to be measured. The resistance of an ammeter is very low, so when it is inserted in a circuit, it causes a small decrease in the current. This is because the ammeter adds to the total resistance of the circuit, and according to Ohm’s law, current is inversely proportional to resistance.
Option A is incorrect because the current will decrease, not increase, when an ammeter is inserted in the circuit. Option C is incorrect because the current will not remain the same, it will decrease. Option D is incorrect because the current will decrease, not remain the same or increase.