For an ellipse $\dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} = 1$ ($a > b$), the **length of the latus rectum** is:

**Latus rectum** is the chord through a focus perpendicular to the major axis. For the horizontal ellipse, plug $x = c$ into the standard equation: $\dfrac{c^2}{a^2} + \dfrac{y^2}{b^2} = 1 \Rightarrow \dfrac{y^2}{b^2} = 1 - \dfrac{c^2}{a^2} = \dfrac{a^2 - c^2}{a^2} = \dfrac{b^2}{a^2}$. So $y^2 = \dfrac{b^4}{a^2}$, $y = \pm \dfrac{b^2}{a}$. Total chord length = $\dfrac{2b^2}{a}$. **Trap.** Option A swaps $a$ and $b$. Option D ($2a$) is the major-axis length, not the latus rectum.