Autotrophs differ from heterotrophs in that only autotrophs
(1) require carbon dioxide for cellular respiration
(2) release oxygen as a product of cellular respiration
(3) synthesize nutrients using carbon dioxide and water
(4) break down sugars to assemble other molecules

The correct answer is (3) synthesize nutrients using carbon dioxide and water.

Explanation:

Autotrophs are those organisms that are able to make their own food. Most of the autotrophs are plants and algae which synthesize their food in the form of carbohydrate like glucose by using carbon dioxide and glucose as precursor.

Energy is required by the autotrophs to make food which is provided by the solar radiation which is absorbed by a pigment called chlorophyll present chloroplast of plants.

These autotrophs are primary producers which are then consumed by heterotrophs that can not make their own food. Therefore only autotrophs synthesize nutrients using CO₂ and H₂O so the right answer is third.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

b.