Chọn phương án C.

Ta có $$\begin{aligned}
\overline{x}&=\dfrac{5\cdot2+15\cdot3+10\cdot4+6\cdot5+4\cdot6}{40}\\
&=3,725.
\end{aligned}$$Khi đó $$\begin{aligned}
s^2&=\dfrac{5\left(2-\overline{x}\right)^2+15\left(3-\overline{x}\right)^2+10\left(4-\overline{x}\right)^2+6\left(5-\overline{x}\right)^2+4\left(6-\overline{x}\right)^2}{40}\\
&\approx1,3494.
\end{aligned}$$Vậy phương sai bằng \(1,35\).