The correct answer is option 4: Dinosaurs were reptiles that became extinct. Dinosaurs were a diverse group of reptiles that roamed the Earth millions of years ago. They were not mammals (option 1), as mammals are a completely separate group of animals that evolved later. Dinosaurs were also not related to hippos (option 2), as hippos belong to a different evolutionary lineage. Additionally, dinosaurs were not egg-laying mammals (option 3), as mammals give birth to live young. The extinction of dinosaurs occurred around 65 million years ago, most likely due to a combination of factors including climate change and an asteroid impact. It is important to note that birds are considered to be the descendants of dinosaurs and are the only surviving group of dinosaurs today.