The cause of the quarrel:

The quarrel mentioned in the question was caused by the poetess not knowing something. This is evident from option 'B' which states that the cause of the quarrel was that the poetess did not know.

Explanation:

When there is a quarrel, it means that there is a disagreement or argument between two or more people. In this particular case, the cause of the quarrel is related to the poetess not knowing something. Let's explore this further.

Understanding option 'B':

Option 'B' states that the poetess did not know. This implies that there was a situation or a question where the poetess lacked knowledge or information. This lack of knowledge became the cause of the quarrel.

Reasons behind the quarrel:

Though the given information is limited, we can speculate on possible reasons for the quarrel based on the options provided.

- a) Their favorite book: It is unlikely that a quarrel would arise solely due to differences in favorite books. While people may have different preferences, it is not a strong enough reason to cause a quarrel.

- c) For money: Money-related issues can often lead to quarrels, but since it is not mentioned in the options that there was any involvement of money, we can rule this out as the cause of the quarrel.

- d) For chocolate: Similar to the previous option, since it is not mentioned that chocolate was involved in the quarrel, we can rule this out as well.

Conclusion:

Based on the given options, the most logical cause of the quarrel is option 'B', which states that the poetess did not know. This implies that the quarrel occurred because the poetess lacked knowledge or information about something, leading to a disagreement or argument. However, without further context or information, it is difficult to pinpoint the exact details of the quarrel.