Which of the following is a living fossil?

The correct answer is Tuatara.

Step-by-step Explanation:

1. Understanding "Living Fossils":
A living fossil is a term used to describe a surviving species that has remained virtually unchanged in its physical characteristics over millions of years, and has few or no close living relatives.

2. Analyzing the Tuatara:
The tuatara (scientific name: Sphenodon punctatus) is a reptile endemic to New Zealand. Although it closely resembles lizards, it belongs to a completely distinct and ancient order called Rhynchocephalia (or Sphenodontia). This group flourished during the Mesozoic Era, around 200 million years ago, alongside the dinosaurs. Over time, all other species within this order became extinct, leaving the tuatara as the sole survivor. Because its anatomy has remained largely unchanged for millions of years, it is classified as a living fossil.

3. Evaluating the Other Options:
- Dinosaurs: Non-avian dinosaurs went extinct approximately 66 million years ago, so they are not living fossils.
- Lizards and Snakes: Both belong to the order Squamata. This is a highly diverse, modern, and successfully radiating group containing thousands of species, meaning they are not evolutionary relics.