Pluralization of [ the apple of one's eye]

Solution 1:

Although I can't provide evidence to support this, my feeling is that "apple of my eye" is not syntactically flexible. The pluralization of 'eye' (Sentences c and d) definitely sounds wrong. The pluralization of apple in Sentence b might be acceptable to some native speakers, but it sounds wrong to me. The only sentence I would consider natural is Sentence a - They are the apple of my eye. I think that here, apple is being used as a figurative term for something general and uncountable, such as a feeling of attachment or devotion. I suspect that is why it sounds strange in the plural.

Maybe an etymological analysis could shed some light on this.

Solution 2:

From Wikipedia,

"The phrase apple of my eye refers in English today to something or someone that one cherishes above all others. Originally, the phrase was simply an idiom referring to the pupil of the eye."

From Quora,

"Your eye (that is, you yourself) can have only one ‘apple’.

The apple of one’s eye is the delight of one’s life, the thing or person he or she loves best in all the world. You COULD say ‘the apple of my EYES’ (plural eyes) but you can’t have more than one MOST BELOVED thing/person.

One apple to a customer."