Answer: A. by observing the formation of sedimentary rocks.
Relative aging is a dating process in which the approximate age of a fossil is deduced by comparing the fossils with layers of rocks where these are found. Therefore, it is an age estimation method that involves estimation of age with respect to the other object. The method of reading the order or sequence of layers of sediments of rocks or soil strata is called as stratigraphy.
On the basis of the above information, relative- age dating used to determine the ages of fossils is done by observing the formation of sedimentary rocks.