A \( \left\{\frac{n(n+1)}{2}\right\}^{2} \)
B \( \frac{\mathrm{n}(\mathrm{n}+1)}{2} \)
C \( \frac{\mathrm{n}(\mathrm{n}+1)}{4} \)
D \( \frac{\mathrm{n}^{2}(\mathrm{n}+1)}{2} \)
Correct Answer: Option B
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