Explanation:

- Dogfish are a group of small sharks found predominantly in saltwater environments.

- Let's evaluate the options given:

- River: Rivers are freshwater habitats. Dogfish are not typically found here.

- Sea: The sea is a saltwater environment and the natural habitat for many species of dogfish.

- Lake: Lakes are freshwater bodies, similar to rivers. Dogfish do not typically inhabit these.

- Marsh: Marshes are wetlands that can be freshwater or saltwater but are not a typical habitat for dogfish.

- The correct answer is Option 2: Sea.