The correct answer is option 4, Chalk. Chalk is a sedimentary rock that is formed from the accumulation of microscopic marine organisms called coccolithophores. These organisms have a chalky outer shell made of calcium carbonate. When the coccolithophores die, their shells sink to the bottom of the ocean, where they accumulate over time and form layers of sediment. Over millions of years, this sediment gets compressed and solidified into rock, creating the chalk formations that we see today.

Options 1, 2, and 3 are not organically formed sedimentary rocks. Shale, option 1, is a fine-grained rock that is formed from the compaction of clay or silt particles. Chert, option 2, is a hard, dense rock that is formed from the sedimentary deposition of microcrystalline quartz. Halite, option 3, is a mineral that is formed from the evaporation of saline water, but it is not an organically formed rock.

Therefore, the correct answer is option 4, Chalk, as it is a sedimentary rock that is formed through the accumulation of organic material.