Final answer:
In the nucleus of a eukaryotic cell, one can find histones, the nucleolus, and various types of RNA such as rRNA and mRNA. Transfer RNA is usually exported to the cytoplasm. Plasmids are not typically found in eukaryotic cells' nuclei but are common in prokaryotic cells, making them the correct answer to the question.
Step-by-step explanation:
The question, 'Each of the following structures is found within the nucleus of a eukaryotic cell except what?' pertains to the various components that are generally found within the eukaryotic cell nucleus. Within this nucleus, one can find histones which are proteins that assist in DNA packaging, the nucleolus which functions in the assembly of ribosomes, and there may be various types of RNA, including ribosomal RNA (rRNA) and messenger RNA (mRNA). However, transfer RNA (tRNA) and other RNAs are transcribed in the nucleus and then exported to the cytoplasm to participate in protein synthesis.
On the other hand, plasmids are typically found in prokaryotic cells, like bacteria, and are small DNA molecules that are physically separate from chromosomal DNA and can replicate independently. It is rare for plasmids to be naturally present in eukaryotic cells, making them the correct answer to the question. Eukaryotic cells, unlike prokaryotic cells, have a membrane-bound nucleus and other organelles that perform specialized functions.