إِنْ يَسْأَلُوا عَنْ سَبَبِ التَّأَخُّرِ ...
If they ask about the cause of the delay ...
In isolation this does not determine the apodosis: “tell them,” “show the letter,” and other responses remain possible. But consider: أَظْهِرِ الرِّسَالَةَ لِلنَّاسِ، وَإِنْ سَأَلُوا عَنْ سَبَبِ التَّأَخُّرِ ...
Show the people the letter; and if they ask about the delay ...
The earlier command can supply an intelligible reconstruction. Still, words not established by context should not be passed off as a certain verbatim answer. Ellipsis is rhetorically effective only where the intended response can be reliably recovered.