The correct preposition to fill in the blank is "into".
The phrase "ploughed back into" refers to the process of reinvesting money into something, in this case, the company.
The other options — "beside", "above", and "through" — do not fit this context.
The correct answer is "into".
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?