When salt A reacts with dilute HCl, it produces pungent gas B. If gas B is passed through alkaline sodium nitroprusside solution then purple solution C is produced. Identify A?

In an alkaline solution containing sulfide ions, the addition of sodium nitroprusside results in the production of the purple sodium trio nitroprusside complex.

The reaction is as follows:

Metal + Acid → Salt + Hydrogen gas

$2Na\left(s\right)+2HCl\left(aq\right)\to 2NaCl\left(aq\right)+H_2\left(g\right)$

Hence, the correct option is D.