The landscape feature labeled x is best described as
(1) a flood plain
(2) a sand bar
(3) a delta
(4) an escarpment

Option (1)

Explanation:

Floodplain is usually defined as the flat, low-lying areas that are present adjacent to a river channel or streams. They are mainly comprised of loose and unconsolidated sediments such as sand, silt, clay and other finer particles. These areas are constructed by the continuous flow of river water and also by the overbank deposition. These areas also attain its feature due to the episodic flooding. The water, when flows over these regions, deposits numerous minerals, thereby increases the fertility that is useful for vegetation.  

Thus, in the given image, the letter X represents a floodplain.

Hence, the correct answer is option (1).