Well, at first we look at this problem and say what, this doesn't make sense. Ionization energy is supposed to increase across periods, not drop! Well, there just so happens to be an anomaly in this all too perfect trend. To better understand this question, look at the visual display of first IEs below.
Also, to take a look at another representation, click here
Now, what the problem is asking for is this: why does IE drop from Mg to Al and P to S (you can ignore Si)?
Well, the answer in a nutshell is that it is a matter of stability. Look at the orbital filling diagrams below: