How would I know which question to pick when sitting my creative writing exam?

Read through all six very carefully. From the first read through, eliminate the scenarios and scenes which you have little to zero knowledge about, and those which you have no vague, but immediate ideas for. Then look at the style of writing which the remaining questions require you to employ. From here, choose the one with which you are most comfortable. If there is more than one, use brainstorming to further narrow it down and therefore, you can pick which question would give you the best results and the most ease. Remember that the paper is timed, and thus you have to be as efficient as you can. The overall planning stage should be approximately 10 minutes of your allocated time; no more.