Which of the following is a biotic factor in an ecosystem?

In an ecosystem, biotic factors are the living components that can interact with one another and affect one another's existence. Symbiosis is a clear example of a biotic factor as it involves the interaction between different species living in close physical proximity, often to the benefit of at least one of the parties involved. This can include relationships such as mutualism, commensalism, and parasitism, all of which significantly influence population dynamics, species interactions, and community structure within an ecosystem.

The other options mentioned—temperature, salinity, and pH level—are considered abiotic factors. These are the non-living chemical and physical components of the environment that affect living organisms and the ecosystems they inhabit. While they play crucial roles in determining the types of organisms that can survive and thrive in a given environment, they do not involve any form of biological interaction between living entities. Hence, these factors are not classified as biotic.