Ammeter should have very low resistance

A tool used to gauge the current flowing across a circuit is an ammeter. It is linked in series with the component of the circuit that will be used to measure the current. This is so because the current in a series combination doesn't change.
A circuit element's current can be measured using an ammeter. As the current through a series combination of elements is constant, it is linked in series with the circuit element. Additionally, the resistances of various components are added together in a series combination.

If the ammeter has a high resistance, it will combine with the resistance of the rest of the circuit and significantly raise the circuit's overall resistance. The influence of the ammeter's resistance on the overall circuit's resistance would be translated into the effect on the current in the circuit because, given a fixed voltage, the current through a circuit is inversely proportional to its resistance. This would imply that the current in the circuit would vary as a result of the ammeter's insertion, making it impossible to accurately measure the current that would have been flowing through the circuit element in its absence. 

If the ammeter's resistance is extremely low, it won't significantly affect the circuit's resistance or current flow.
Ammeters have a very low resistance due to this.

Hence, the correct answer is option D.