Complex organisms with gills and lungs were major life forms that existed in a. mesozoic era c. precambrian era b. cenozoic era d. paleozoic era

Paleozoic is the correct answer

The correct answer is option D, Paleozoic Era

Explanation:

Paleozoic Era is the oldest  era  of evolution that existed between 540 Million to 248 Million Years Ago. During this era, multi celled living things acquired hard body parts , they got bones and teeth  and also the vertebral organs originated during this era. Some common organism that occurred during this era were - fish, insects, amphibians, early reptiles, trilobites, crinoids and brachiopods. The amphibians and fish possessed fins while the reptile possessed lungs.  

Thus Complex organisms with gills and lungs were major life forms that existed in the  Paleozoic Era