Which rock cannot be found as a chemical sedimentary rock?
a. chert
b. limestone
c. sandstone
d. rock gypsum

The correct option is C.
Sandstone is a type of sedimentary rock that is formed by physical weathering of parent rocks. It is usually made up of sand size particles of minerals, rocks, other organic materials and the cementing material that bind the sand grains together. 
Sandstone can not be found as a chemical sedimentary rock because it is formed by mean of physical weathering and not by chemical reactions of the rock. 

Answerhighlands and forests;