Which environmental phenomenon involves the introduction of species by humans?

The correct choice focuses on the introduction of species by humans, a phenomenon commonly referred to as "introduced species." This occurs when organisms are transported and established in a new environment outside their native range, often due to human activity such as trade, agriculture, or intentional release for purposes like pest control or ornamental gardening.

Introduced species can significantly alter the ecosystems they enter, impacting local biodiversity, competition for resources, and interactions with native species. They can sometimes become invasive, outcompeting native species and leading to ecological imbalances, which makes understanding this phenomenon crucial in the study of environmental science and conservation biology.

The other options pertain to different environmental issues: species extinction involves the loss of species, ocean acidification relates to changes in ocean chemistry resulting from increased carbon dioxide levels, and photochemical smog refers to air pollution created through sunlight-driven chemical reactions. Each of these concepts impacts the environment in distinct ways but does not specifically address the introduction of species by humans like the phenomenon of introduced species does.