The chief idea which the author aims to convey through the given passage would be that '<u>invisible ink is not actually invisible</u>.' This claim is substantiated through the descriptions asserting the type of inks(lemon juice, brown ink) that have been used to write such invisible content and also regarding how this invisible content can be visible(by holding the paper over candle's flame or using a chemical). This shows that the ink is not actually concealed completely. Thus, <u>option A</u> is the correct answer.
