What is the smallest lithostratigraphic rock unit? formation bed layer group

Option (1)

Explanation:

The basic lithostratigraphic unit of rock is formation. A formation is comprised of definite rock body whose rock characteristics can be compared to another rock strata, and can be mapped easily.

A bed is defined as the distinctive rock layer that can be differentiate from its upper layer and the lower layer, and are usually more than 1 cm in thickness.

A layer is defined as a distinct rock body, which has a particular rock materials and are distinguishable from one type to another and they are precisely traceable.

A group of rock is comprised of different rock units or layers, forming a division of the geological rock history.

Thus, the correct answer is option (1).