Option 1 states that Silent Valley is the only evergreen forest in India. An evergreen forest is a forest that has green foliage throughout the year. While Silent Valley does have evergreen elements, it is not the only evergreen forest in India. There are several other regions in India with evergreen forests as well.

Option 2 states that Silent Valley contains costly timber trees. While Silent Valley does have valuable timber trees, it is not the main characteristic or highlight of the region. Silent Valley is known for its diverse range of rare and endangered species, not specifically for its timber resources.

Option 3 states that Silent Valley is a good picnic spot. While Silent Valley is undoubtedly a beautiful and scenic place, it is primarily a protected area and a wildlife sanctuary. It is not a designated picnic spot and is subject to certain rules and restrictions to preserve its ecological balance.

Option 4 is the correct answer. Silent Valley is renowned for its rich biodiversity and is home to several rare and endangered species of plants and animals. It is an important conservation area and is recognized as a national park to protect its unique ecosystem.