Final answer:
The untrue statement about phonemes is that different phonemes can never be almost physically identical; phonemes can be acoustically similar and their perception is influenced by context.
Step-by-step explanation:
The question inquires about characteristics of phonemes and their context dependency. The statement that is untrue about how phonemes are ambiguous and context-dependent is option B, which claims that different phonemes can never be almost physically identical. In fact, phonemes can be acoustically similar and can vary based on their context, which is known as coarticulation. Moreover, phonemes can sometimes be inaudibly inferred by the listener based on the surrounding phonetic context, a phenomenon known as phoneme restoration. We indeed cannot splice out phonemes to create clear speech, as their pronunciation is influenced by the phonetic environment.