The pH range of given indicators are
i) bromothymol blue : 6 (yellow) to 7.6 (blue) : so it cannot be used for the given condition
ii) litmus : it senses the change between acid and base. blue litmus turns red in acidic medium (from basic or neutral) however here both the solutions are acidic so we cannot use it
iii) bromocresol green : the pH range is 3.8-5.4. Below 3.8 it is yellow and above 5.4 it is blue
thus we can use it for two solutions as
a) in solution of pH = 3.5 it will show yellow colour
b) in solution of pH =5.5 it will show blue colour
hence answer is Bromocresol green
iv) thymol blue: pH rang is 6.0 - 7.6, so cannot be used
