An idiomatic expression is a type of informal English where the meaning
of the expression is different than the literal meaning of the
expression.
From the options, when 'apple' is used to fill the blank, the phrase 'apple of her eye' is an idiom that refers to 'something or someone that one cherishes above all others'. Hence option A is the most suitable option.
When the words ball, sugar and candy are placed in the sentence, it does not contribute to the intended meaning. 'Ball of her eye' has no complete meaning, same goes for sugar.
(Also, 'candy of her eye', or 'eye candy', refers to a person who is very good to look at, but has little value in the eyes of the beholder. It isn't relevant to the text here.)
Hence options B, C and D are incorrect.