Answer:
b). Standard ruling
Step-by-step explanation:
Context is characterized as the situational background or circumstances in which a specific event takes place that helps clarify the meaning of the event or occurrence. Similarly, the words that are placed in a text before or after an unfamiliar word help clarify its meaning to the readers.
In the given passage, the word 'principle' denotes the meaning 'standard ruling' as per the context. The author conveys that he had been unable to find a standard rule regarding books on which everyone agrees. He expresses that it had been different for different individuals as some are useful for some people while others for others. Thus, option b is the correct answer.