Ais a flat, triangular-shaped land formed when a river carries and deposits sediment at its mouth. question 11 options:

The answer is "Delta"

Delta is a flat triangular shaped land formed when a river carries and deposits sediment at its mouth.
This usually happens where a river is entering an ocean, sea, lake, reservoir, 
and another river that cannot carry away the sediment supplied by the river. The last one happens very rarely (Entering a river).

d. i., ii., iii., and iv.