Ans.
Gill slits or pharyngeal slits are defined as opening structures to throat, pharynx, or fish gills, by which passage of water from pharynx to exterior takes place in fishes. In primitive chordates, gill slits help in filtration of particles of food from surrounding water. In amphibians and fishes, gills are used for the gaseous exchange. In chordates that live in lands, such as mammals, gill slits found in developing embryos only.
Thus, the correct answer is 'option D). all chordates' as gill slits are present in all chordates at some stages of their development.