Question

which is the only national park in the world​

Answer 1

Despite its small size, Nairobi National Park features a large and diversified wildlife population including four of the Big Five animals of Africa and is considered both a lion and rhinoceros sanctuary.

Answer 2

There are more than 4,000 national parks currently present in the world, so giving a one national park is not possible, but there can be answers if asked on basis of some data :

Largest National park - Tassili N Ajjer

Smallest national park - Moyenne Island National Park

An only national park located in capital city of any country - Nairobi National Park

and many other endless answers can be there.