If you can choose which task to do you must make this decision quickly but carefully. Read all the tasks and choose the one which is the easiest for you or one you are most interested in. You will write better English if you are interested in the topic. You can see all different types of writing tasks that are often used in exams in our Writing skills practice section. If possible, try to find out before the exam what type of writing you are going to have to do and if you have a choice, make sure you feel prepared to write several of the different task types.
Do you ever get a choice of tasks when you do writing exams? Which task do you usually choose?