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Vertebrate tetrapods are amphibians. They evolved during the Devonian period and were among the earliest tetrapods found terrestrially. Aquatic tetrapods of the Devonian period include Ichthyostega and Acanthostega.

No proofs have been found that suggested the Acanthostega laid shelled eggs. Hence, option (d) is the correct answer.

a. It was aquatic.

Fossils were found which suggested they gills so they were aquatic. Hence, option (a) is the incorrect answer.

b. It had gills.

Fossil findings showed they had gills. Hence, option (b) is the incorrect answer.

c. It had four limbs.

Fossil findings showed they had four limbs with skeletal structure of limbs similar to that of present-day tetrapods. Hence, option (c) is the incorrect answer.