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".
Each question has a sentence that has an underlined part. Choose the option which rephrases the underlined part in the most appropriate manner in terms of grammar and usage.
In an attempt to boost their profits, many edible oil-producing companies have been engaging themselves in propaganda against commonly used oils and promoting exotic and expensive varieties of oils \(\underline{as\ more\ healthier\ options.} \)