Concept:The sentence requires a gerund phrase acting as the subject of the verb "has."
Explanation:The blank must be filled by a word that can begin a noun phrase serving as the subject.
"Being" is the present participle of "be," but here it functions as a gerund, meaning it acts like a noun.
A gerund can introduce the subject of a sentence: "Being rich has its own problem" is grammatically complete.
"Been" is the past participle of "be," so it cannot act as a noun or subject in this position.
The options "On being" and "On been" wrongly insert the preposition "on," which is not needed before the subject.
No other option forms a correct, meaningful sentence.
Thus, "Being" is the only valid choice.
Answer:A. Being