While students are studying a rock outcrop along the highway they discover a sedimentary rock layer that contains fish fossils what best explains why these fossils are there

The earth crust is made of tectonic plates. the plates are pieces of crust that slowly move across the planet's surface over millions of years. The sedimentary rocks and the fossils originated in the bottom of an ocean and then were transported by the plates over millions of years until they reach their current destination near the highway.

The fossils are there because it is likely that the particular area that the students are in was once covered by water. It could have either been fresh water or salt water. If the area is near the ocean then it was likely once covered by the ocean.