Single celled, no nucleus, and live everywhere; can be beneficial or disease-causing.

• A. Virus
• B. Fungus
• C. Protozoa
• D. Bacteria

Answer: (D)

Organisms that are single-celled and lack a nucleus are prokaryotes, with bacteria being the classic example. They’re found in almost every environment on Earth—from soil and water to the human body. Some bacteria are beneficial: they help digest food, synthesize vitamins, support nitrogen cycling, and are used in fermentation and biotechnology. Others can cause disease by producing toxins or invading tissues. The other options don’t fit because viruses aren’t cells at all and are acellular; fungi are eukaryotic and have nuclei (even when some are single-celled, they still possess a nucleus); protozoa are also single-celled but contain a nucleus. So the description best matches bacteria.