Argue, as a key word, asks you specifically to take a position and defend it. The best arguments have a narrowly focused position statement, reasons to support the overall position, and then evidence to support each reason. If you have time, you can also look at other possible positions and support (again with evidence) why your position is better.
Writing tip: Most students have little trouble stating their overall position, but in the heat of writing under pressure students do often forget to give adequate evidence to support that position. Be sure to include not just general reasons why you hold the position but also the evidence--the details, examples, analysis--that supports your reasons. If you think of a solid argument like a house, you can't hold up the roof (overall position) with a frame (reasons for the position). And you surely can't keep out the rain without the substance (details) that covers the frame.
Specific advice for OT students: Not all arguments need to take a long time to develop. If you need to justify a particular intervention, sometimes a few details and a reference to a pertinent theoretical framework will suffice.
Substitute key words: defend, take a stand or position, justify