While the company's initial proposal promised significant cost savings through automation, it has since been revealed to contain several unrealistic assumptions, causing concern among investors.
Correct: D
The original sentence (Option A) is grammatically acceptable and the pronoun 'it' clearly refers to 'the company's initial proposal.' The participial phrase 'causing concern among investors' correctly modifies the entire preceding clause (the revelation of unrealistic assumptions). However, Option D offers a more precise and impactful phrasing. By replacing 'causing concern' with the appositive 'a revelation that has caused concern among investors,' Option D explicitly states *what* caused the concern – the 'revelation' itself, rather than implying the assumptions directly cause the concern (though the revelation of them certainly does). This phrasing is more precise and emphatic, a characteristic often valued in GMAT for clarity and conciseness where meaning is enhanced. Option B uses an unnecessary past perfect 'had promised.' Option C uses 'as containing,' which is less idiomatic than 'to contain,' and 'which causes' changes the meaning and tense. Option E retains the less precise 'causing concern' and less idiomatic 'as containing.'