Final answer:
The most prominent feature of Mars observable from Earth is its bright polar caps, which can be seen through a telescope and change with the seasons.
Step-by-step explanation:
The most prominent feature of Mars observable from Earth is its bright polar caps. When viewed through a telescope, the bright polar caps on Mars are clearly visible and change with the Martian seasons, resembling the seasonal snow cover on Earth.
These caps are particularly noticeable because, at the resolution available from Earth-based telescopes (about 100 kilometers), topographic structures such as mountains, valleys, and even impact craters are not detectable.
However, the polar ice caps can be easily seen, along with dusky surface markings that sometimes change in outline and intensity from season to season.
Other surface features, such as the large volcano Olympus Mons and the canyon system Valleys Mariners, are known from spacecraft but are not discernible with ground-based telescopes at this resolution.