Concept:A sentence completion question cannot be solved without the full sentence that contains the blank.
Explanation:Each option is a grammatical form of the verb "hear": "to hear" (infinitive), "for hearing" (preposition + gerund), "at hearing" (preposition + gerund), and "on hearing" (preposition + gerund).
Each of these could fit correctly in a different type of sentence.
Because the actual sentence is missing, the intended meaning and grammatical structure are unknown.
Therefore, no single option can be identified as the best completion.
Answer:Cannot be determined from the provided information.