the option is A because we know that the new median is 5 after adding one more number to the series 2, 3, 5, 7 (even terms). After you add one more term lets say x to the series, we get odd terms so the term in between is the median of the given numbers and that is possible only when the number added is greater than or equal to 5 so that the terms in ascending order become 2,3,5,5,7 or 2,3,5,x,7 or 2,3,5,7,x in all these cases the median is 5 but if the term is less than 5 i.e. 4.8 then we get 2,3,4.8,5,7 and hence 4.8 becomes the median.
So the only term that cannot be added for the median to be 5 is 4.8 i.e. option A.