Tafsir of Ghafir 40:81

"And He shows you His signs," meaning His indications that point to the perfection of His affair, may His Majesty be glorified. "So which of the signs of Allah do you deny?" meaning, which sign among those manifest signs do you deny?

For each one of them is of such clarity that anyone possessing even a modicum of reason would barely dare to deny them. Thus, "which" (ayya) is for interrogative rebuke, and it is in the accusative case governed by "do you deny" (tunkirun). The genitive construction of "the signs" to the Majestic Name is to cultivate awe and to emphasize the gravity of denying them.

Indefining "which" (ayya) in such a context is the common and widespread usage, while its feminization is rare. Among the instances of the latter is the saying: By which book, or by which Sunnah, do you consider love for them a shame upon me, and you reckon...

Al-Zamakhshari said: "This is because distinguishing between masculine and feminine in nouns other than adjectives—such as himar (donkey) and himarah (she-donkey)—is strange. In the case of ayya, it is even stranger because of its inherent ambiguity, for it is an interrogative noun used for that which is vague and unknown to the inquirer. Such a distinction contradicts the nature of the word, as it implies differentiation between what is feminine and masculine, which would render it known to him."