The correct answer is option 3: Lion-tailed macaque. Silent Valley is home to many rare and endangered animal species, but the lion-tailed macaque is particularly notable. This species is native to the Western Ghats region of South India and is characterized by its distinctive manes of hair and black fur. They are considered endangered due to habitat loss and fragmentation caused by human activities such as deforestation and agriculture. The lion-tailed macaque is an important species to conserve as it plays a crucial role in maintaining the biodiversity of the ecosystem. It is worth noting that the other options in the question (musk deer, tiger, and rhinoceros) are also endangered species found in different parts of the world, but not specifically in Silent Valley.