The correct answer is A) 300. The population of northern right whales, an endangered marine mammal species, is approximately 300 individuals. This low population number is due to the species' history of overexploitation by commercial whaling, which nearly drove them to extinction. Current conservation efforts have helped stabilize the population, but it remains critically endangered. The other answer choices (B, C, and D) vastly overestimate the actual population size and are therefore incorrect.