To be a possible answer to a scientific question, a hypothesis must be testable. unique. written. controlled.

The best answer would be:

TESTABLE

By definition a hypothesis is a testable, educated prediction of what would happen in an experiment. It is a prediction that can be proved or disproved. If it does not possess this one quality, it overall is not a hypothesis. A hypothesis is not a statement or a claim. You need to try it out to see if it is true before it can answer any question.

As to the other choices, not all hypotheses are unique. The reason why it is testable, is to ensure that the hypothesis would stand the test of time and development. It is not necessarily written and nor is it controlled.

"d.) carbon dioxide and hydrogen sulfide"