Which best blocks x-ray particles and can therefore be observed more closely through the use of this technology?

a. organs
b. muscles
c. fluids
d. bones

D. Bones

Explanation:

The X - ray has a penetration power trough the skin, tissues and muscles. But the bones are so hard that the energy of X -ray can not penetrate and is reflect, that is the reason why we can observe the bones in a radiography.  

i guess that visual aids present persuasive examples to what you are presenting, some factors would be theme (does it fit with the color, information, what you are presenting), size (can you see it, is it a big enough span of data/information), and overall look of the visual aid.

hope this : )

positive reinforcement? ?