B. 1/2
Explanation: 1/4 of grape juice as already in the bottle and then a mixture containing orange, grape and apple filled the bottle meaning the mixture was 3/4. If the mixture contained equal amounts of each juice it means 1/3 of each juice was in the mixture so 1/4 plus 1/4(the original amount of grape juice that was in the bottle before the mixture was added) is equal to 2/4 simplify that and you have 1/2
