Hi Abhilash,
I must admit that you have a rather challenging task on hand and that is also the reason why you must make a good attempt at taking it on with optimism and determination.
Let me share with you a few points on how I interpret the two terms and hopefully you will get some pointers on how to build up your assignment.
Reasons:
They are proactive responses
Have a logical and rational build up
Offer an outline to re work / salvage a situation
Seek to explain rather than justify
Excuses:
Are an escapist mechanism
Twist facts to suit the situation
May ignore inconvenient truths
Focus is on justifying rather than on getting solutions
I am sure you can think up of a few more features of both and then elaborate on each point.
Best Wishes for your presentation.