Which of the following statements about meridians of longitude is correct?

The correct answer is B. Every meridian runs from the North Pole to the South Pole, so all meridians are semicircles of the same length and all of them converge at the two poles. That is exactly what distinguishes them from parallels.

Option A describes parallels, not meridians: parallels are full circles that never meet. Option C is wrong in both directions, since meridians are equal in length and it is the distance between them, not the meridians themselves, that changes; the gap is widest at the equator at about 111 kilometres and falls to zero at the poles. Option D is a near miss that examiners like: a single meridian is only half a circle, and it is a meridian taken together with its opposite meridian that forms a complete great circle. The equator, by contrast, is a great circle on its own.