Let's ask ourselves this question: Which variable depends on another variable's quantity?
In this problem, the number of birthday candles Vincent buys is dependent on his Grandmother's age. If we were to set up an equation to represent this situation, it would look something like this: b = y + x. Therefore, since we know that the number of birthday candles is dependent on another variable, it cannot be independent. The independent variable is y, Vincent's grandmother's age in years.
Hope this helps! :)