At what latitude of the Earth can you see: a) all the stars of the celestial hemisphere at any moment of the night;

At what latitude of the Earth can you see:
a) all the stars of the celestial hemisphere at any moment of the night;
b) stars of only one hemisphere (northern or southern);
c) all the stars of the celestial sphere?

a) At any latitude, at any moment half of the celestial sphere is visible;
b) at the poles of the Earth, the northern and southern hemispheres are visible, respectively;
c) all the stars of the celestial sphere can be seen at the equator of the Earth in less than a year.