The correct option is
B Zenith
We would be choosing a word opposite to the given word, "Nadir".
Nadir means the lowest or most unsuccessful point in a situation.
If we look into the other options,
Progress means forward or onward movement towards a destination
Liberty means the state of being free within society from oppressive restrictions imposed by authority on one's way of life, behaviour, or political views
Zenith means the time at which something is most powerful or successful. It is the exact opposite of Nadir
Modernity means the quality or condition of being modern