What is the total number of electrons shown in this lewis structure of carbon dioxide?
8
12
16
24

C. 16

Note that for this explanation all electrons mentioned are those that appear on valence shells only.

You can look at it in two ways:

Carbon has 4 electrons, oxygen has 6. You have 1 carbon atom and 2 oxygen atoms.
Add them together, and you get a total of 16.

Or you can look at it this way:

Refer to the Lewis structure of CO2.
Each oxygen has 2 lone pairs.
2 oxygens * 2 lone pairs per oxygen * 2 electrons per pair = 8 electrons
There are 4 bonds total in the Lewis diagram: 1 double bond for each carbon to oxygen.
4 bonds * 2 electrons per bond = 8 electrons
Add those together, and you get a total of 16.