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I think your issue may be that you are reading the questions to fast and missing out on those little nuances which change the answer, although the 2nd question is pretty badly worded.
For the 1st question:
You're asked what decreased the underlying defecit, decreasing the defecit means government payments are exceeding receipts by a lesser amount than before. By process of elimination, the medibank option although an increase in receipts is one off and thus does not effect the underlying defecit. Interest rates have no direct effect on the budget outcome.
Those ones you should eliminate immediatly and then you're left with two. Option C increase payments, which increases the defecit. However option D increases receipts through increased taxation, reducing the defecit, hence is the correct answer.
For the 2nd question:
What the question is essentially asking (in very bad wording), is what happens to the budget outcome when economic growth is increasing and government makes no discretionary changes. A we eliminate immediatly as it talks about the discretionary section of the budget and the question specifically excludes such items. B is incorrect as the defecit will decrease in times of increased growth (as those relying on welfare such as unemployed decrease and government starts collecting more incomes taxes). C again talks about the structural outcome which is irrelevant. D is correct as the decreased defecit as a result of economic growth increasing decreases the net aggregate demand the government is contributing to the economy, hence reducing economic activity.
Advice:
For MC you need to fully read the question and break down what it is asking. For most questions you should determine the answer before you even read the available options. For example if a question asks "what happens to the cyclical component of budget in times of decreased economic growth", I can immediatly dejuce the correct answer would involve something which decreases receipts and increases payments in the budget, hence reducing a surplus or increasing a defecit. Highlighting key words in the question and subsequently available answers is a great way to make sure you fully read and understand the question.