I wrote 3a and 3c...I thought they were pretty easy to write based on my experiences.
For 3a I focused on all the outside demands on my time that made the academics much harder than they should have been, as well as how I took lots of courses outside my comfort zone because of my school's largely standardized curriculum. Without directly mentioning my relatively low quant score, I noted all the technical and quant courses I had taken and done well with, saying that my past success gives me confidence that I can handle that type of work in the future.
3c is tailor-made for an "I had this type of problem/frustration in my career, and here's how I dealt with it and overcame it and was productive anyway" kind of approach. Mine dealt with me being disappointed based on unrealistic expectations I had when I started my current job, but how I found ways to improve the situation for me and the organization rather than just being miserable and dragging along.
That was just my approach--obviously any of them can work. If you have a really spectacular career vision that will distinguish you from the masses, go for that. If you have any good stories about being thrown into situations as an outsider yet were still successful forming cohesive bonds with your peers, effective as a leader and a follower, etc. you could write the introduction one pretty easy.