Given that Joan has "q" quarters, "d" dimes, and "n" nickels in her pockets, you can determine that the total number of coins she has in her pockets is the sum (the result of an Addition) of all the quarters, dimes, and nickels she has in it.
Therefore, knowing that the given variables represent the number of each type of coin, you can write the following Addition of variables, in order to represent the total number of them:
Hence, the answer is: Option a.