Assertion (A) : The Equator is a great circle. Reason (R) : The Equator encircles the whole Earth and its plane divides the Earth into two equal halves.

The Equator is defined as a great circle because it is the intersection of the Earth's surface with a plane that passes through the center of the Earth [t4, t7]. By definition, a great circle is the largest possible circle that can be drawn on a sphere, and its plane divides the sphere into two equal halves [t3, t8]. While all meridians (lines of longitude) form great circles when paired with their opposite meridian, the Equator is the only line of latitude (parallel) that qualifies as a great circle [c1, t9]. Other parallels of latitude are considered 'small circles' because their planes do not pass through the Earth's center and they do not divide the Earth into equal halves [t4, t9]. Therefore, the Reason correctly explains why the Equator is classified as a great circle, making both statements true and the Reason the correct explanation for the Assertion.

Sources

1. [1] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Great Circle Routes > p. 14
2. [2] https://education.nationalgeographic.org/resource/great-circle/