Final answer:
The difficult topic that St. Gregory of Nyssa discusses in relation to God's providence is option D) predestination.
Step-by-step explanation:
According to the given excerpts and information, the difficult topic that St. Gregory of Nyssa discusses in relation to God's providence is predestination.
The concept of predestination is the belief that God has predetermined the eternal destiny of every individual, whether they will be saved from the time of their birth. This belief has been discussed and debated by various theologians throughout history, including St. Gregory of Nyssa, John Calvin, and Augustine.
St. Gregory of Nyssa's discussions on predestination reflect his theological views and insights into the relationship between God's providence and human free will.