On the occasion of World Environment Day 2021, Raimona was declared as a National Park in India.

Option 1: Raimona - This is the correct answer. Raimona was declared as a National Park on World Environment Day 2021. It is located in the state of Assam and is known for its rich biodiversity.

Option 2: Orang - This option is not the correct answer. Orang National Park is already an existing national park in Assam, but it was not declared as a National Park on World Environment Day 2021.

Option 3: Dibru-Saikhowa - This option is not the correct answer. Dibru-Saikhowa National Park is located in the state of Assam, but it was not declared as a National Park on World Environment Day 2021.

Option 4: Nameri - This option is not the correct answer. Nameri National Park is also located in Assam, but it was not declared as a National Park on World Environment Day 2021.

In conclusion, Option 1 (Raimona) is the correct answer as it was declared as a National Park in India on the occasion of World Environment Day 2021.