These seem to be exam questions, which aren't allowed here. Your best source for the answers would be to go to Google or Yahoo and look up "stress testing" and "requirements testing". If you have questions after you've learned what they are, come back here and tell us what you think the answer to number 3 is and we can help.
1 = The purpose of a stress test is to find out what the capacity limits are for a system, which is a critical input for setting throttles on how much activity to accept at once. You also want to know if exceeding the normal performance targets (tested in performance testing) leads to graceful degradation rather than a system crash.
2 = You never test for errors in a software product, you test for compliance with requirement specifications. Testing may drive out errors due to hard failures or due to non-compliance, which is quite different. Ideally your QA efforts work towards preventing errors in the first place and scoring a high error count is an SDLC process failure, not a success for QA or testing.
3 = You need to know precisely WHAT you try to validate. Software testing is the means to an end and not an end in itself. Do you know where to find that information?
... and the answer to the 3rd question usually requires a book of which many are referenced here at this site. That question is probably somewhat answered in your study materials and extensively in the numerous threads of discussion at this site. Whichever resource you decide to use, any or all require that you study and further challenge yourself. Those acts alone build the discipline and skills you need to be in this industry.
I don't know if I would say you test the requirements and never test for errors. A great percentage of testing is 'negative testing'. Testcases that people never envisioned as being part of the requirements. These are usually left up to the expertise of the tester. Question #3 can't really be answered here. This would be the whole software development lifecycle. *IF* this is really an exam question, it must be a trick question because you can't know based upon that information. Some projects require stress testing and some don't.