Which principle or law of geology states that in a sequence of undeformed sedimentary rocks, each bed is older than the one above?

Answer: Law of Superposition

The law of superposition related to the strata in geology (also called Steno's Law) states that "sediment layers are deposited in a temporal sequence, in which the oldest are in a position inferior to the most recent"

This principle was initially proposed in the 11th century by the Persian geologist Ibn Sina, and was first proposed as a law in the 17th century by the Danish scientist Nicolás Steno.