Final answer:
The tadpole stage of the amphibian life cycle prevents them from being fully terrestrial, as they are aquatic and rely on gills for respiration.
Step-by-step explanation:
The part of the amphibian life cycle that prevents them from being fully terrestrial is the tadpole stage. During this stage, amphibian larvae, such as tadpoles, are aquatic and breathe using gills, resembling fish with their long-finned tails and lack of limbs. Amphibian eggs must be laid in water to prevent drying out, as they do not produce amniotic eggs. These eggs are protected with a jelly-like substance that keeps them moist. As amphibians undergo metamorphosis from the larval stage to the adult form, they develop limbs, lungs for air-breathing, and adaptations that prepare them for terrestrial life. However, until this metamorphosis is complete, they are bound to the aquatic environment due to their initial physiological makeup and developmental requirements.