Which of the following compounds is added to the sodium extract before the addition of silver nitrate for the testing of halogens?

In the test for halogens,why is nitric acid added to the sodium fusion extract before adding silver nitrate?

In the Lassaigne's test for nitrogen in an organic compound,the Prussian blue colour is obtained due to the formation of:

Three students,Manish,Ramesh,and Rajni were determining the extra elements present in an organic compound given by their teacher. They prepared the Lassaigne's extract $(L.E.)$ independently by the fusion of the compound with sodium metal. Then they added solid $FeSO_4$ and dilute sulphuric acid to a part of the Lassaigne's extract. Manish and Rajni obtained prussian blue colour,but Ramesh got red colour.
Ramesh repeated the test with the same Lassaigne's extract,but again got red colour only. They were surprised and went to their teacher and told him about their observation. Teacher asked them to think over the reason for this. Can you help them by giving the reason for this observation? Also,write the chemical equations to explain the formation of compounds of different colours.

Lassaigne's test for the detection of nitrogen fails in

Which one of the following sets of elements can be detected using sodium fusion extract?