# To do with qualitative analysis?

Problem:

When a colourless solution S was added to aqueous sodium hydroxide, no visible change was seen. When aluminium powder was added, a pungent gas which gave brown precipitate when bubbled into aqueous iron(III) chloride was obtained. What ions could be present in S?

(a) Na+, SO42-
(b) K+, NO3-
(c) Na+, Br-
(d) Ca2+, NO3-

Attempts:

The answer is (b), but I have no idea how to arrive at the answer. I was thinking about ammonium ions since ammonia (pungent gas) would be produced when sodium hydroxide and aluminium were added to ammonium ions, but there was no answer. I simply do not understand.