6. Arrange the stones in the shape of a trapezoid as shown in the figure, and call the sequence of numbers $\left\{a_{n}\right\}: 5,9,14,20$, ... formed by the number of stones in the "trapezoid" structure the "trapezoidal sequence". According to the construction of the "trapezoid", it can be known that $a_{624}=$
A. 166247
B. 196248
C. 196249
D. 196250
$$
\begin{array}{l}
a_{1}=5, a_{n+1}-a_{n}=n+3 \Rightarrow a_{n}=\sum_{k=1}^{n-1}\left(a_{k+1}-a_{k}\right)+a_{1}=\sum_{k=1}^{n-1}(k+3)+5 \\
=\sum_{k=1}^{n-1} k+3(n-1)+5=\frac{n(n-1)}{2}+3 n+2=\frac{n^{2}+5 n+4}{2}=\frac{(n+1)(n+4)}{2} .
\end{array}
$$

Then we have $a_{624}=\frac{625 \times 628}{2}=625 \times 314$, so the answer is $D$.