Final answer:
Line 16, "Great lord of all things, yet a prey to all;" best illustrates the poet's use of contrast to depict life's essential conflicts through the juxtaposition of power and vulnerability.
Step-by-step explanation:
The best line that supports the claim that the poet uses contrast to illustrate the essential conflicts of life is d) "Great lord of all things, yet a prey to all;" (Line 16). This line clearly depicts a stark contrast between being a 'great lord' and being 'a prey', which represents the duality of human existence and the inherent conflicts within life itself. This juxtaposition effectively captures how humans can feel in control and powerful while simultaneously being vulnerable and subject to outside influences or harm.