فَبِأَيِّ آلَاءِ رَبِّكُمَا تُكَذِّبَانِ
Which of your Lord’s favours will you both deny?
The question in Ar-Raḥmān 55:13 is not a request for a list of favour-names. Against the surrounding descriptions of divine gifts, it challenges denial and invites acknowledgement.
The initial fa links each occurrence to surrounding discourse; bi-ayyi frames the challenge through the favours named. The wider passage is indispensable to reading its rhetorical function.