Sorry i was so late i guess i can't do it now.
The Cornell method has been proven to be very useful & is used throughout high school/college education. color coding notes & making them aesthetically pleasing can impact your focus on the info. flash cards or repetition is helpful in the memory process
Going by now it got better because back then there was a depression and economic failures and then we had got out of the depression and we still somewhat are having economic failures but i hope that helps.
B, handwritten letters, because the letters are probably the most credible, especially since they're a first-hand account of things that happened to your ancestors, and as WorldlyGlass45 mentioned in their answer to this question on another post, you'll most likely get the most info from the letters as well.
It would be C because well, is it not obvious.