For the standard form of a **horizontal** ellipse $\dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} = 1$ with $0 < b < a$, the value $a$ represents the:

In a horizontal ellipse with $a > b$, the larger denominator $a^2$ sits under $x^2$, so the major axis lies along the $x$-axis. The semi-axis along the major axis has length $a$ — i.e. $a$ is the **semi-major axis**. The semi-minor axis has length $b$ (option B); the focal distance is $c = \sqrt{a^2-b^2}$ (option C); the latus rectum length is $2b^2/a$ (option D).