The subordinate clause is "you must start researching now"
Out of your 3 possible answers, B. ones' is the correct one and I will explain why. Answer A is a contraction of one and is. When you put "one is" into the blank space you can tell that it is not right. Answer C is incorrect because when you place an apostrophe after an s it is implied that there should be another s that follows and so you do not need to put it there. This means that answer B is correct since it doesn't have the extra s.
