أَمْ لَهُمْ سُلَّمٌ يَسْتَمِعُونَ فِيهِ
...
Or have they a stairway, by means of which they listen.
meaning, do they have a stairway to heaven (to the place where the angels are),
...
فَلْيَأْتِ مُسْتَمِعُهُم بِسُلْطَانٍ مُّبِينٍ
Then let their listener produce some manifest proof.
meaning, let their listener produce evidence to the truth of their actions and statements. They will never be able to do so and therefore, they have nothing and have no evidence for their stance.
Allah admonishes them regarding their claim that He has daughters and that the angels are females! The pagans chose for themselves male offspring and preferred them instead of females, and when one of them would be brought the good news of a daughter being born, his face would turn dark on account of his suppressed anger! Yet, they made the angels Allah's daughters and worshipped them besides Allah,
أَمْ لَهُ الْبَنَاتُ وَلَكُمُ الْبَنُونَ