Final answer:
The youngest organism from the illustration is likely the fern, as it is part of a lineage that includes species existing today, unlike the trilobite, crinoid, and ammonite, which have no direct descendants alive in the present.
Step-by-step explanation:
The student has asked which organism from the illustration is the youngest. The four options provided were the trilobite, the fern, the crinoid, and the ammonite. To answer this question, we must look at the geologic time scale and the fossil record.
The trilobite is known to have been one of the most dominant species during the Cambrian period and disappeared at the end of the Permian period. As for the fern, it is a type of plant that has been present on Earth for hundreds of millions of years, with many species still living today. The crinoid is a type of echinoderm that dates back to the Ordovician period, but some species still live in modern oceans. Lastly, the ammonite is a type of cephalopod that first appeared in the late Silurian and became extinct at the end of the Cretaceous period.
Without the specifics of the era each one is from in the illustration, it's not possible to accurately rank their ages. However, if we assume that each of these specimens is a representative of when these organisms were most prominent in the fossil record, the likely answer would be the fern, as they are part of a line that includes descendants existing today. Therefore, the fern would be the youngest organism in comparison with the trilobite, crinoid, and ammonite, all of which have no surviving descendants (in the same form) and are only known from fossils.