Answer: I would say The hypothesis that the juice of the lemon is the only pat of lemon that is capable of removing stains from the t-shirt is because the lemon juice is acidic in nature.
1.) To test the validity of this hypothesis he should test the acidity of different parts of the lemon including the juice, pulp and skin of lemon.
2.) He should also compare the lemon juice with other juices to know weather other juices are also helpful in removing stains from the t-shirt.
( I hope this helped <3 )