The correct answer is B. Th phrase that would provide the best supporting details would be "the idea of a convention devoted to women’s rights was born when".
This is so because this phrase would allow the writer to include a series of data or additional information, which would serve to give more strength to the postulates enunciated by the main text.
It is that the supporting details are not necessarily an integral part of what is written, but are accessory questions to this that allow a better understanding or knowledge of the text.
Thus, in this case, it is proposed to explain the origin of a certain question (and not the question itself), with which the sentence is the starting point for the development of supporting details in the text.
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