Acceptance criteria are the conditions that must be met for a solution or deliverable to be accepted by the stakeholders. Acceptance criteria should be measurable, testable, and clear, so that the team can verify if the requirements have been met or not. If the acceptance criteria for the requirements were not measurable, the team would have difficulty confirming if certain requirements have been met, as they would not have a clear and objective way to evaluate the solution or deliverable. The other options are not likely to cause this issue.
The test engineer is not responsible for communicating requirements to the team, but for testing them against the acceptance criteria. Not all stakeholders need to be involved during requirements elicitation, as long as the key stakeholders who can provide relevant and accurate information are consulted. Requirements should be adequately reviewed with the project sponsor, but this does not affect the confirmation of meeting the requirements, as the sponsor is not the one who verifies them. References: PMI-PBAExamination Content Outline, page 13; PMI-PBAReference List, page 1, BABOK Guide v3, page 39.