This sounds like a great, interesting project!
1. For the hypothesis, I think that would work fine. Perhaps you would want to rank them on which would hold up the most weight- as in, prism would hold up the least and triangular pyramid would hold up the most, or whichever order you decide on. That would lead to the most easily testable hypothesis.
2. For testing, I think that (a) would work the best- I think some of the weight would likely be distributed to the columns, and that the plank or columns could even give way before the shape. That might change the results- to get the best data, I'd recommend the first method.
The only concern that I have is that it seems a little difficult to stack sand bags on a pyramid; some might slide to the side, or fall off. Perhaps you could set up a small platform out of cardboard if the weights fall off?
If you still have questions or need any more help, please don't hesitate to ask!
Hope this helps,
Nature uses only the longest threads to weave her patterns, so that each small piece of her fabric reveals the organization of the entire tapestry.