Does the biosphere have clear boundaries

No, the biosphere does not exhibit a clear boundary as life always comes to an end.  

The biosphere comprises all the living things of the world, like all the plants and animals. It is considered as the global ecosystem, that is, the gathering of all the planet's ecosystems. It comprises the land and the atmosphere. It is usually considered as the zone of life, where all the species of the planet thrives.  

the answer is d.) potassium