Final answer:
The transitional form linking aquatic and terrestrial lobe-fins is Acanthostega, an early tetrapod that had both gills and lungs and limbs like modern tetrapods, but was not fully adapted for life on land.
Step-by-step explanation:
The extinct transitional form linking aquatic and terrestrial lobe-fins is c. Acanthostega. It is one of the earliest known tetrapods that lived in the Devonian period. Acanthostega possessed traits that were intermediate between fish and early amphibians, such as gills and lungs, as well as limbs with skeletal structures similar to those in modern tetrapods. However, its limbs were not yet capable of supporting the animal's weight on land, suggesting a life mainly in shallow freshwater environments with possible brief terrestrial excursions. While the coelacanth is a fascinating lobe-finned fish that was once thought to be extinct and related to the transition from aquatic to terrestrial life, it is not considered a transitional form itself but rather a member of the Sarcopterygii clade that has remained relatively unchanged. The true intermediate clade between fish and amphibians is evidenced by the discovery of Tiktaalik roseae, a "fishapod" that preceded Acanthostega and had a mix of features from both finned fish and legged tetrapods.