How do I attempt the unseen comprehension passage to score full marks?

Reverse your order. First skim the passage once to get the general idea, then read the questions, and only then read the passage again carefully, underlining lines that answer each question. This way you read with a purpose. Answer in your own words where possible, but for fact-based questions you may quote the relevant line. Write complete sentences, not one-word answers, unless the question allows it. For vocabulary questions, find the word in context rather than guessing from memory. For 'why' or inference questions, look at the surrounding sentences for clues. Always attempt every question, since there is no negative marking, and keep your answers within the marks allotted to manage time.