Assess the view that the military planning of the great powers, rather than their long-term rivalries, better explains the outbreak of the First World War in 1914.

This question is great because you've already been given two factors which you should discuss! As a basis I try to address factors in my essays while simultaneously presenting counter-arguments, so let's have a look at a mock plan:

Point 1: Military Planning, here I would refer to the Schielffen plan involving mobilisation which was hard to stop once set into motion.  I would also refer to the work of Taylor and his War by Timetable to back up this argument. It could be worth incorporating French ideas of high morale & rapid advance offering a tempting solution to problems. These plans in the wider context made war seem like an option to address long-standing anxieties. It is important though to remember that planning for war alone is unlikely to explain an outbreak of war. 

Point 2: Long-Term Rivalries, this will require more background knowledge. It could be useful to mention French resentment about the loss of Alsace Lorraine & defeat in 1870, Russian concern with the Balkans & dangers of German-backed Austria & Austrian fear of long-term viability of their Empire. But as a counter-argument it would be worth mentioning whether these rivalries would have led to war without a confident military plan. 

What we've learned:

1) Address the factors provided in the question

2) Incorporate counter arguments into your essay

3) ALWAYS form an argument, examiners love it and it helps you structure your thoughts!