The correct answer is option C. balance.
The balance can be used to calculate the mass of apple. In this case, she need to take a normal apple, which grow in untreated soil and one apple, which grow in treated soil, she should make sure, the apples are of similar size and shape. by using balance she can measure the mass of apple and determine the difference between masses of two apples.
The graduated cylinder is used to detect change in volume, so it is not useful.
The beaker is used to hold liquids, it cannot measure anything, so it is also not useful.
The pH meter determine pH of the substance, hence, it is also not useful.