External fertilization is characterized by which of the following?

External fertilization is characterized by the release of gametes into an aquatic environment, which is the correct answer. In this process, both sperm and eggs are released into the water, where fertilization occurs outside of the bodies of the parents. This method is common in many aquatic species, such as fish and amphibians, allowing for the sperm to swim to the eggs in the surrounding water, facilitating fertilization.

The other options do not accurately describe external fertilization. The direct introduction of sperm into the reproductive tract pertains to internal fertilization, where sperm are deposited within the female's body. Involvement of terrestrial species only is misleading since external fertilization typically occurs in aquatic environments. Lastly, the statement about gametes remaining within the female body also aligns with internal fertilization processes, where fertilization happens internally rather than in an external environment.