Well as you said, for an ideal transformer
\dfrac{I_s}{I_p}=\dfrac{N_p}{N_s}
Hence,
I_s=I_p\dfrac{N_p}{N_s}
Now, for the galvanometer to deflect the most, the current produced has to be the highest, this only happens when I_s is big, so either when N_p is increased or N_s is decreased. Hence the...