A sentence is given with an underlined part. Choose the alternative that corrects the error, or select "No improvement": "None knows how it was happened."

Introduction / Context:
This sentence improvement question focuses on correct verb form and word order in English. The sentence given is: None knows how it was happened. There is a clear grammatical error involving the auxiliary was and the past participle happened. Your task is to pick the option that produces a natural and correct English sentence while preserving the intended meaning: nobody knows how the event took place.

Given Data / Assumptions:

• Subject: None, meaning nobody or no one.
• Verb phrase in the that clause: how it was happened, which is ungrammatical.
• The event is completed, so some form of past tense is needed.
• The options present different combinations of auxiliary verbs and past forms.

Concept / Approach:
The verb happen is intransitive and does not normally take a passive form with be. We say the accident happened, not the accident was happened. Therefore, the correct structure in the clause about the event is simply how it happened, using the simple past. We do not insert was before happened because that would create an incorrect passive form. The main clause None knows can remain in present simple to express a current state of ignorance about past events. As a result, the best correction is to replace was happened with happened.

Step-by-Step Solution:
Step 1: Focus on the embedded question how it was happened and identify the verb happen. Step 2: Recall that happen is intransitive and does not form a passive with was. The natural pattern is how it happened. Step 3: Consider Option C, happened, which gives the clause how it happened, fitting the pattern. Step 4: Reject Option A, was happen, because was must be followed by the past participle happened, not the base form happen. Step 5: Reject Option B, has happened, because how it has happened would change the nuance and is less natural in this kind of indirect question about a past event.

Verification / Alternative check:
Compare the corrected sentence with other standard sentences. We say No one knows how it happened, Nobody knows how the fire started, and None knows how it ended, all using simple past in the embedded clause. Using a passive form such as how it was happened simply does not occur in natural English because happen itself expresses an event occurring, not an action done to an object. Similarly, the present perfect has happened is usually used with a specified present relevance (for example, I do not know how this has happened), not with the general indirect question pattern here. Therefore, the best and most common structure remains how it happened.

Why Other Options Are Wrong:
was happen: This is wrong because was must be followed by a past participle, and happen is not the correct form. The phrase also incorrectly suggests a passive construction where none is needed.
has happened: This is grammatically possible but awkward in this context. None knows how it has happened sounds less idiomatic and introduces a present perfect nuance that is not required in a general statement about ignorance.
No improvement: This is wrong because the original phrase was happened is ungrammatical and not used in standard English.

Common Pitfalls:
A frequent mistake is to overuse passive structures with was or were plus a past participle, even when the verb is intransitive and cannot logically take a passive voice. Learners also sometimes choose present perfect forms just because they look sophisticated. To avoid these errors, remember that verbs like happen, occur, and take place describe events that simply happen and are not done to something; they normally appear in active simple past in sentences like the one in this question.

Final Answer:
The sentence should be corrected to None knows how it happened.