Step 1: Concept
In an Arithmetic Progression (AP) of three terms $a, b, c$, the middle term $b$ is the average of the first and last terms: $b = \frac{a+c}{2}$.
Step 2: Meaning
We want to find $S = a+b+c$.
Step 3: Analysis
Statement I only gives $a+c=4$. Statement II tells us $b = (a+c)/2$. Combining them, we find $b = 4/2 = 2$. Then $S = (a+c) + b = 4 + 2 = 6$.
Step 4: Conclusion
Neither statement alone can find the sum, but together they do.
Final Answer: (C)