Step 1: Concept The least upper bound (Supremum) is the smallest value that is greater than or equal to all elements in the set.
Step 2: Meaning Let $a_n = \frac{3n+2}{2n+1}$. We test the first few terms: for $n=1, a_1 = \frac{5}{3} \approx 1.66$; for $n=2, a_2 = \frac{8}{5} = 1.6$; for $n=3, a_3 = \frac{11}{7} \approx 1.57$.
Step 3: Analysis The function $f(n) = \frac{3n+2}{2n+1}$ is a decreasing function because as $n$ increases, the terms get smaller and approach $1.5$.
Step 4: Conclusion Since the sequence is decreasing, the first term ($n=1$) is the largest value in the set. Thus, the least upper bound is $\frac{5}{3}$.
Final Answer: (C)