from your knowledge about the distribution of electrons in their shells and from the atomic number (in parentheses), indicate the most likely charge on the ion when this atom forms an ion. (remember the 2-8-18 shell distribution.)

oxygen (8)

+1
+2
-2
-1

The correct option is -2.

Explanation:

Oxygen atom will have two electrons in its first shell and six electrons in its outermost, valence shell. To become stable, an oxygen atom will need to gain two electrons so that it has 8 electrons in its outermost, valence shell. When oxygen will gain two electrons from another atom, it will get a charge of -2. This is because it has acquired two more negative charged electrons. Hence, the correct option is -2.

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.

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms