Pluralization of [ the apple of one's eye]

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.

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."

You could say: "When I feel depressed, I hang out with my family; They are each an apple of my eye, and as such I am a person who breaks with normal idiomatic expressions by having many apples of my eye."

First some etymology: The phrase was coined in the KJV Bible and used in subsequent translations:

Keep me as the apple of your eye; hide me in the shadow of your wings (Psalm 17:8, ESV)

keep my commandments and live; keep my teaching as the apple of your eye; (Prov 7:2, ESV)

For thus said the Lord of hosts, after his glory sent me to the nations who plundered you, for he who touches you touches the apple of his eye (Zechariah 2:8, ESV)

In the KJV and subsequent translations (such as the modern ESV), "apple of your/his eye" means the pupil, meaning a tender, vulnerable, and guarded part of the eye. So in Psalms and Zechariah there is a plea that the Psalmist or the nation of Israel might be reflexively protected by God.

One could argue that in the Zechariah verse, the you is plural. (I will have to ask my resident Hebrew scholar to verify.) Even many people are still a single apple. The pattern for a plural would be They are the apple of my/his/your eye. Thus, probably the best rendering would be choice (a):

they are the apple of my eye.

(I inserted the before apple to preserve the parallel with the original usage as well as the cadence of the KJV.)

In The Hiding Place, Corrie Ten Boom's father also uses the form They have touched the apple of my/his/your eye:

"Father! Those poor people!" I cried. . . .

"Those poor people," Father echoed. But to my surprise I saw that he was looking at the solders now forming into ranks to march away. "I pity the poor Germans, Corrie. They have touched the apple of God's eye.”

(The Hiding Place) (Although this is a recent publication from the 1970s, note that Ms. Ten Boom was likely translating from Dutch into contemporary English).

We're dealing with idioms, therefore it's what sounds right.
I'd say something like this is entirely comprehensible to speakers of the language that know the original expression:

"My two twin boys, yep, they're the apples of my eye."