The type of charge an object possesses can be determined by observing its interactions with other objects of known charge. To make such a determination, you need to combine the observations with three simple rules regarding charge interactions:

• Objects that possess opposite types of charge will be observed to attract each other. 
• Objects that possess the same type of charge will be observed to repel each other. 
• A neutral object and a charged object will always attract each other.

It's All About Reasoning
Like a lot of our Concept Builders (but maybe more so with this one), you have some thinking and logical reasoning to do.  The physics rules are straight-forward:

• opposites attract,
• likes repel, and
• charged and neutral attract.

But the application of these simple rules can get complicated. Get yourself organized and prepare to reason.


The Game Plan
This question is about Balloon C. The charge of Balloon A is stated.  But Balloon C does not interact with Balloon A; it only interacts with Balloon B.  So you must draw some conclusions about Balloon B from its observed interactions with Balloon A.  Ask yourself if the diagram displays attraction or repulsion between A and B. Then look up the corresponding rule - they're listed in the paragraph above - It's All About Reasoning. It's important to note that there are two rules for explaining why attraction occurs. There's only one rule for repulsion. There's more on the logical significance of this in the next two paragraphs.


Repulsion Provides Evidence for Clear Conclusions
There is only one reason for why two objects repel - they have the same type of charge. Neutral objects can never be involved in repulsion. So if you observe that Balloons B and C repel, you know that they are both charged and charged with the same type of charge. The repulsion of Balloons B and C rules out any possibility of B or C being neutral. They are both charged with the same type of charge. Now look at Balloon B's interaction with Balloon A in order to determine what type of charge it has.
 

Careful about Attraction
There are two reasons for why two balloons attract. They could be oppositely charged or one could be charged and the other is neutral. The observed attraction of Balloons A and B by itself is not conclusive enough evidence that B is charged. But when you observe how Balloon B interacts with Balloon C (discussed above), you can rule out the possibility of Balloon B being neutral and narrow in on the conclusion that is it charged and charged opposite of Balloon A. Now use that information to make a conclusion about the charge of Balloon C.