The correct answer is B: flawed. The word "rejected" indicates a negative reason for the proposal's unacceptability. A flawed proposal is one that contains errors, weaknesses, or imperfections that make it unsuitable or unworkable. These flaws could be in its logic, data, feasibility, or potential outcomes, rendering it unacceptable to the manager. Managers typically reject proposals that do not meet certain standards or present significant risks or deficiencies, and 'flawed' perfectly encapsulates such reasons.
Let's consider why the other options are incorrect. A: practical means sensible and realistic, which would be a reason to accept, not reject, a proposal. C: beneficial implies that the proposal would bring advantages or positive results, making it highly desirable and likely to be approved. Similarly, D: useful suggests that the proposal serves a purpose or is helpful, which would also be a strong argument for its acceptance. All these alternatives describe positive attributes that would lead to approval, directly contradicting the action of rejection. Therefore, 'flawed' is the only option that provides a logical and appropriate reason for a manager to reject a proposal, highlighting its inherent deficiencies.