We always use ammeter in series to calculate the current drawn by the element from voltage source. If the net resistance of an ammeter is high, because of the series connection it will add up the net resistance of an ammeter. Then we can’t get the current values accurately. If net resistance of an ammeter is low, it only shows a very small changes in current to be measured those can be negligible.

The net resistance of the ammeter should be small. Option D is correct.