Final answer:
The correct FANBOYS conjunction to use in the given sentence is 'but,' as it shows the contrast between the father's instruction and the student's action.
Step-by-step explanation:
In the sentence provided, an appropriate FANBOYS conjunction to use would be 'but,' because it indicates contrast between two ideas. In this case, the contrast is between what the dad told the student and what the student actually did. The complete sentence would then be 'Dad told me not to play on my iPad, but I was playing on it.'