In which reaction do two compounds exchange their ions or radicals mutually to form two new compounds?
Double displacement reaction.
The correct option is B Double displacement reaction.
Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions. In double replacement reactions, the positive ions and negative ions switch their positions and form two new compounds or products.
$$KOH + HNO_3\rightarrow H_2O + KNO_3$$
$$Na_2S + 2HCI \rightarrow 2NaCl + H_2S$$