Which one of the following is the longest parallel of latitude ?

The Equator (0° latitude) is the longest parallel of latitude because it represents the Earth's maximum circumference [2]. As an oblate spheroid, the Earth's width is greatest at its center; consequently, the length of parallels decreases as one moves polewards from the Equator toward the North and South Poles. While the Equator is a "great circle" that divides the Earth into two equal halves, all other parallels, such as the Tropic of Cancer (23.5° N), the Tropic of Capricorn (23.5° S), and the Arctic Circle (66.5° N), are "small circles" with progressively smaller circumferences. For instance, the 60th parallel is only half the length of the Equator. Therefore, the Equator is the only parallel that achieves the maximum possible length on the terrestrial grid [2].

Sources

1. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Longitude > p. 11