Well, given that a quarter equals 5 nickels, we can reason that, in order to get 12 coins, we would have to get 10 nickels (equals 50 cents) and 2 quarters (the other 50 cents, to make a dollar).
You didn't say whether you needed this expressed in a mathematical formula, but knowing the answer already, it shouldn't be hard to come up with one if necessary.
The correct answer is the graph in the top right corner because it is a constant function. The bottom left graph is also considered a function but is an absolute function. So both are technically correct.
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