Amanda's gingiva is puffy and red, indicating that she's not removing plaque properly. what would the hygienist use to make the plaque visible to amanda so she can learn how to remove it properly? a. an x-ray b. a dentifrice c. a disclosing solution d. mouthwash

Because it is asking for a teaching technique don't think that an x-ray would show Amanda her plaque.  A dentrifice is a polishing solution used for after a teeth cleaning, that would not show Amanda her plaque.   A normal mouth wash would not show Amanda her plaque.  
The only answer that would show Amanda where her plaque is is a disclosing solution!  They even sell it for kids as mouth wash so they can make sure they get all the plaque off when they brush.
So C. a disclosing solution is the answer.

"observation" would be the correct answer