Final answer:
The sister group to tetrapods is lungfish, belonging to the lobe-finned fishes (Sarcopterygii), which share similar characteristics such as rudimentary lungs.
Step-by-step explanation:
The sister group to tetrapods is the Sarcopterygii clade, which includes lobe-finned fishes like lungfish and coelacanths. Among these, the more closely related group to tetrapods are the lungfish, as they share several physiological characteristics, such as a rudimentary lung structure, with early tetrapods. This makes lungfish the most closely related to tetrapods among the extant fish groups, meaning the correct answer is option (c) lungfish.