Final answer:
A Russian prisoner likely plays a small, portable instrument, such as a concertina or harmonica, for Paul to hear in an act typical of cultural moments in literature and historical scenarios.
Step-by-step explanation:
The question refers to a scene where a Russian prisoner plays an instrument next to the fence for Paul to hear. This scenario is reminiscent of moments in literature and film where music provides solace or communication in dire circumstances. Although the specific work – and thus the exact instrument – is not mentioned in the question, we can infer from the context and similar narratives that harmonica-like or small accordion instruments, such as a concertina, were commonly depicted in such scenes due to their portability and prevalence among soldiers and prisoners of war.