Step 1: Understanding the structure.
The sentence is in the past modal form, expressing possibility or uncertainty about a past event. For this, we use “might have + past participle (V3).”
Step 2: Analyzing the options.
- (a) What might have happened to the boy? — Correct; follows the rule “might have + V3.”
- (b) What might has happened... — Incorrect; “might” is a modal and cannot be followed by “has.”
- (c) What might has happening... — Grammatically wrong; incorrect tense.
- (d) What may has happened... — Incorrect; same error with “has” after a modal.
Step 3: Grammar Rule.
Modals such as “might,” “could,” and “may” are always followed by a base verb or “have + past participle” form when referring to the past.
Step 4: Constructing the correct sentence.
“What might have happened to the boy?” correctly expresses speculation about a past event.
Step 5: Conclusion.
Thus, option (a) is grammatically correct.