Final answer:
Luther could be unhappy due to the conditions of his 'cell', such as lack of sunlight or loneliness, influenced by Plato's allegory of the cave, or it could be related to Martin Luther's disapproval of the Church's practices. Context is necessary for a definitive answer.
Step-by-step explanation:
Without the specific context regarding Luther, it's difficult to provide a concrete answer. However, if Luther refers to Martin Luther, our understanding based on the information provided in Chapter 2 could point out to his dissatisfaction with the mandates of the Church. If we relate Luther to a hypothetical prisoner inspired by the allegory of the cave by Plato, his unhappiness could stem from the harsh change and disorientation after his release from the 'cell' or cave (b), and perhaps a mental anguish which could be interpreted as loneliness (d). We should note that these are only educated deductions and further context is needed to narrow down the correct answer.
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