Break it down in arrows and see the magic of simplicity.
Remember:
Only if A then B means A is required for B
This can be written as B implies A or B -> A
which means if B then A has definitely occurred but if A, then B may or may not occur
Let's get back to the question =>
Independent Learners (IL) are required for Teacher Effectiveness (TE) --- TE -> IL
Teachers' Power (TP) is required for Student Decisions (SD) --- SD -> TP
Student Decisions (SD) are required for Independent Learners (IL) --- IL -> SD
We are given these 3 conditions and then the conclusion is Teachers' Power (TP) is required for Teacher Effectiveness (TE) --- TE -> TP
Now if you pause here and think, then you will realize that by combining the premises we can easily arrive to our conclusion ie.
TE -> IL -> SD -> TP
Question: Given SD, each could be true EXCEPT?
So everything left to SD is a sufficient condition whereas everything right to SD is a necessary condition which means for SD to occur TP should have happened but we can't comment anything about IL or TE.
It's like saying - If it rains today then we will go to park (Rain -> Park). Now if I say that we went to park today, then did it rain today? May be or may be not. Let's say we just wanted to enjoy cool breeze during the evening and we went for a stroll. But now if I say it rained today, then we can definitely say that we went to the park today. ie. going to park is required if it rains but if we go to park then that doesn't mean it rained today.
So similarly given SD, we can confidently say that TP occurred but we can't say anything about TE or IL.
Now check the options, you now shouldn't need more than a sec to select the write choice.
(A) Their students have not become independent learners. (~IL)
(B) They are not effective teachers. (~TE)
(C) They are effective teachers. (TE)
(D) They have the power to make decisions in their own classrooms. (~TP) => Correct
(E) They do not have the power to make decisions. (TP)