The correct option is C a and d
Many species on earth are declining or becoming extinct due to various human activities. Habitat loss and fragmentation, overexploitation, alien species invasions and co-extinction are the four major causes for the decline or extinction of species. Steller’s cow (Russia) and passenger pigeon (North America) are two species whose extinction is a result of overexploitation by humans.
Steller’s sea cow was hunted by sailors and traders for meat. Overkilling of this aquatic animal led to its extinction.
Many passenger pigeons were slaughtered for many years and they were sold in the market which caused them to become extinct eventually. Population of many marine fishes is also declining across the world because of overharvesting.
Intentional or unintentional introduction of alien species into certain habitats leads to interspecific competition between the alien and the native species which can result in extinction of the native species. Clarias gariepinus and Lantana are examples of alien species.
Lantana is native to tropical regions of South America and is an invasive species that was introduced in India by British as an ornamental plant. It competes with native plants for space and resources, and also alters the nutrient cycle in the soil. It invades agricultural landscapes, forests and wastelands and harms commercial interests of farmers and foresters.
African catfish (Clarias gariepinus) is native to Africa. It was introduced in India for aquaculture but now poses a threat to the indigenous fishes.