Final answer:
Thomas, the cost accounting manager, violated the ethical principle of credibility by not disclosing that his initial assumptions were impractical and could affect the investment's profitability.
Step-by-step explanation:
In the given scenario, Thomas, the cost accounting manager of Daisy Inc., has violated the ethical principle of credibility. Credibility in the professional field of accounting and business includes, among other things, providing all relevant information that could influence a user's understanding or decision. By realizing that his initial assumptions were impractical and choosing not to disclose this vital information to the management, Thomas compromised the credibility of his analysis. Ethical behavior requires that all relevant and material information is communicated transparently, allowing for informed decision-making.