The correct option is A Invisibly
Words that are similar to each other in meaning are also known as synonyms of each other. The word ‘Imperceptibly’ means something that is so slight or subtle that it cannot be perceived or seen beforehand. While 'Inconceivably' is tangentially related to the word, and both ‘Intensely’ and 'Extremely' are irrelevant to the word in terms of meaning, 'Invisibly' is the closest in meaning to ‘Imperceptibly’ – leaving us with the correct answer, Option B).