Answer:
b. The internal resistance must be much smaller than the other resistances in the circuit.
Explanation:
Ammeter is used to measure the current flowing through a circuit. It is connected in series configuration with the load. In such a scenario the resistance of the ammeter should be negligible so as to make sure that the voltage drop across the resistance of ammeter is zero and it shows the correct reading of the current in the circuit.
For a standing wave on a string, the wavelength is equal to twice the length of the string:

In our problem, L=50.0 cm=0.50 m, therefore the wavelength of the wave is

And the speed of the wave is given by the product between the frequency and the wavelength of the wave: