In this English grammar sentence improvement item, you are given the sentence The reason why the magazine sells so well is because it is written in a pleasant style, which contains an underlined expression that may be redundant. Choose the alternative that removes the grammatical fault and produces a more concise but accurate sentence, and choose No improvement only if the original structure is already fully correct.

Introduction / Context:
This question focuses on the correct way to link a reason clause with the word reason in English. Examinations often test the common error of using both why and because together in the same clause, which creates redundancy. The sentence talks about why a magazine sells well, and the underlined part is is because. The goal is to choose the connector that makes the sentence grammatically correct and stylistically clean.

Given Data / Assumptions:

• The original sentence is The reason why the magazine sells so well is because it is written in a pleasant style.
• The structure reason why ... is because is suspected to be faulty.
• The options suggest for, since, that, and No improvement.
• The context is formal standard English suitable for examinations and textbooks.

Concept / Approach:
In English, the noun reason can be followed by different structures, but we should avoid redundancy. The pattern the reason is that is considered correct, and so is the reason why is, but combining why and because together often creates a double expression of cause. The phrase reason why already contains the idea of cause, and because introduces cause again, so standard usage avoids putting them together in this way. Therefore, we look for an option that replaces because with a more suitable linking word, turning the clause into a clear complement of the verb is without repeating the sense of cause unnecessarily.

Step-by-Step Solution:
First, identify the core structure the reason why the magazine sells so well is because it is written in a pleasant style. Second, notice that both why and because refer to the idea of reason, so the phrase may sound awkward and redundant. Third, recall that a standard alternative is the reason why the magazine sells so well is that it is written in a pleasant style. Fourth, check the answer choices and see that using that after is solves the problem by introducing a complement clause rather than another explicit causal conjunction. Finally, confirm that the revised sentence is smoother and acceptable to exam level grammar: The reason why the magazine sells so well is that it is written in a pleasant style.

Verification / Alternative check:
You can test the sentence by removing why and keeping because, which gives The reason the magazine sells so well is because it is written in a pleasant style. Many modern style guides still view reason is because as undesirable. On the other hand, The reason the magazine sells so well is that it is written in a pleasant style is widely accepted. Grammar references frequently recommend the structure the reason is that rather than the reason is because. Searching reliable corpora or usage guides will confirm that the pattern reason is that is safe and standard in formal prose.

Why Other Options Are Wrong:
Option A, for, would give the reason why the magazine sells so well is for it is written in a pleasant style, which is ungrammatical because for cannot directly introduce a finite clause in this way.
Option B, since, would produce is since it is written in a pleasant style, which also sounds clumsy; since is more natural at the beginning of a sentence and not after the verb is in this specific structure.
Option D, No improvement, keeps the original redundancy and goes against recommended formal usage, so it is not the best answer.

Common Pitfalls:
A frequent learner mistake is to combine several causal expressions together, such as the reason is because, or the reason is due to because, which all repeat the same idea. Another pitfall is to assume that conversational patterns automatically count as correct in examinations, even though exam boards often follow more conservative grammar rules. Learners should train themselves to recognise that the reason is that is an accepted pattern and that adding why or because in the same structure may be marked as wrong in many tests.

Final Answer:
The redundant and less acceptable combination is because should be replaced with a simple complementiser that, so the correct choice is that.