Final answer:
The letter 'A' likely represents the Mercator projection, commonly used in textbooks. However, in practice, other projections such as the Azimuthal Equidistant or Polar Stereographic are preferred for polar regions.
Step-by-step explanation:
The letter that represents the map projection often used to show polar regions is A, corresponding to the Mercator projection. However, this may be a trick question because the Mercator projection is not typically best for polar regions due to distortion at high latitudes. Instead, a projection such as the Azimuthal Equidistant or Polar Stereographic would often be used, but these are not options provided in the question. If the intention is to identify a projection commonly used in textbooks and other sources for illustrating the Earth, then A (Mercator projection) is most likely the answer they are aiming for based on this context.