Hi Akash,
Option A is less appropriate because the phrase "as being" is unnecessarily wordy and does not align well with formal usage, particularly in contexts requiring precision, such as historical or scientific writing. Additionally, "a reasonable certainty" is slightly awkward, as "reasonable certainty" is typically treated as an uncountable concept, making the article "a" unnecessary.

Option D is the better choice as it is concise and grammatically precise. The use of "to be" establishes a direct and clear connection between the estimation and the age, ensuring the sentence flows naturally and aligns with formal conventions.

I hope this helps.