Which one of the following sedimentary rocks are organically formed?

Sedimentary rocks are classified based on their mode of formation into mechanical, chemical, and organic (biochemical) types [1]. Organically formed sedimentary rocks result from the accumulation and lithification of plant or animal remains [1]. Chalk is a prime example of an organically formed calcareous rock, primarily composed of the microscopic remains (coccoliths) of marine organisms like coccolithophores [2]. While limestone can be formed organically or chemically, chalk is specifically identified as biogenic [1][3]. In contrast, shale is a mechanically formed (clastic) rock resulting from the compaction of clay and silt [1]. Halite is a chemically formed evaporite that precipitates from supersaturated solutions [1]. Chert can be biogenic (from radiolarians or diatoms) or chemical, but chalk is the most definitive organic example among the provided options.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > 13.2. Sedimentary Rocks or Detrital Rocks > p. 171
2. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > SEDIMENTARY ROCKS > p. 19
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > Organically Formed Sedimentary Rocks > p. 172