Question:
How is it to reply to salam when doing wudhu?

Answer: حامدا ومصليا
If one is occupied in (reciting) the supplications of wudhu, then it is best not to say salam or reply to it.

And Allah Subhanahu Ta’ala knows best

Written by (Faqeehul Ummah Mufti) Mahmood (May Allah have mercy on him)
Darul Uloom Deoband
Fatawa Mahmoodiyah Vol.8 Pg.129 Tarteeb Jadeed