The central thesis of the passage is that class-size reductions: A. can improve academic achievement under some conditions but may not be the most effective way to do so.
B. improve academic achievement when teachers use techniques well suited to small classes.
C. improve academic achievement only in the early elementary grades.
D. are less efficient than other means of improving academic achievement.

A). can improve academic achievement under some conditions but may not be the most effective way to do so.

Explanation:

As per the given passage, the key proposition or the viewpoint that the author conveys is that 'Class-size reduction would help enhance the academic performance under certain circumstances yet it is not the most fruitful way of promoting academic achievement.' The author puts forward his stand over the reduction in sizes of the class that it will promote the effective interaction among the teacher and students that would eventually lead to enhance learning of the students. However, he offers the other side of the issue that it is not the most helpful way for enhancing academic achievements that establishes his credibility. Hence, option A is the correct answer.