When an atom of oxygen forms an ion, what is the most likely charge for that ion? a. -2
b. -8
C. +2
d. -6

-2

How many electrons does Oxygen need:

We know that oxygen has an electric configuration of:

2 , 6     [8 electrons]

From the octet rule, we know that all elements try to get to the electronic configuration of the nearest noble gas

In the case of Oxygen, the nearest noble gas is Neon with the electronic configuration:

2 , 8     [10 electrons]

From the above, we can see that oxygen needs 2 more electrons in order to attain the nearest noble gas configuration

What will be the Charge on the ion?:

We know that electrons are negatively charged

Once oxygen gains 2 electrons to attain noble gas configuration, it will have 2 more electrons than it should have

So, the charge on the ion will be -2 since there are 2 extra electrons and both of the electrons contribute a negative charge

Hence, the charge is -2

it is b that is the answer.