Explanation

(c)
Detection of sulphur in sodium extract is done by lead acetate and sodium nitroprusside
$Na_{2}S + \left( CH_{3}COO \right)_{2}Pb \rightarrow PbS + 2CH_{3}COONa$
lead acetate black ppt.
$Na_{2}S + Na_{2}\left\lbrack Fe(CN)_{5}NO \right\rbrack \rightarrow Na_{4}\lbrack Fe(CN)_{5}NOS\rbrack$
Sodium nitroprusside sodium thio nitroprusside
(purple colour)