Original solution of salt or- mixture should not be prepared in concentrated ${\mathrm{HNO}}_3$ because it:

Since nitric acid ${\mathrm{HNO}}_3$ is an oxidising agent and will change any low oxidation cations into higher ones.  It is often not used to prepare the original solution in the study of basic radicals. This is because these cations cannot be recognised in the presence of the acid.

Hence correct option is b, d