Are you using an IDE to work out the snippets of code while studying ?If yes, then you will have a bit of trouble identifying which line will have a compilation error. So quit using IDE and use a text editor.
If one of the options is a "Compilation fails" then always check for a Compilation error when you start answering, it saves a lot of time. If you cant find one then go for the rest of the options.
Your scores are low, so just postpone taking the tests after you have revisited the concepts. Try to identify your problem areas and then get all the concepts clear. If suppose you are making mistakes in loops, try a number of questions in that alone until you are confident and have mastered it. Try to have a strategy for the areas that you need to improve... Keep track of the time. Make sure you finish the mock exams on time.
My view is don't overlearn the questions from the same book. You will just end up memorizing the answers involuntarily, giving you a false idea that you are good at it.Refer different books/guides outside your comfort zone.
Any mistake, no matter how trivial will cost you.So maybe you should practice on being more attentive.Go through the stories of the ranchers who have taken the exam.This is firsthand info, so will be extremely useful.All the best.