Which term refers to organisms whose cells contain a nucleus?

• A. Eukaryote
• B. Prokaryote
• C. Retrovirus
• D. Nitrogen fixation

Answer: (A)

Organisms with cells that contain a nucleus are eukaryotes. In these cells, the genetic material is enclosed within a membrane-bound nucleus, and the cells also contain other membrane-bound organelles like mitochondria (and, in plants and algae, chloroplasts). This contrasts with prokaryotes, which lack a true nucleus—their DNA sits in the cytoplasm in a region called the nucleoid and they don’t have membrane-bound organelles. A retrovirus isn’t a cell type at all—it’s a virus that lacks cellular structure and a nucleus. Nitrogen fixation is a metabolic process, not a cell-type label, though some nitrogen-fixing organisms are prokaryotes. So the term is eukaryote.