The correct answer is C) keep the answering pennant at the dip. When a receiving station is unable to distinguish a signal sent by flag hoist, the proper procedure is to keep the answering pennant at the dip, which indicates that the signal is not clearly understood. This is in accordance with standard maritime communication protocols. The other options are incorrect because: A) raising and lowering the answering pennant would indicate that the signal is understood, which is not the case here; B) hoisting ZL would indicate a general failure to understand the signal, rather than specifically indicating an inability to distinguish it; and D) hoisting ZQ would indicate a request for the signal to be repeated, rather than indicating an inability to understand it.