b
Alkyl fluorides can be prepared by action of mercurous fluoride or antimony trifluorides (inorganic fluorides) on corresponding alkyl halide. This reaction is known as Swarts reaction. 

\(C H_{3} B r+A g F \rightarrow \underset{\text { Methyl fluoride}}{C H_3F}+A g B r\) 

But, when aciton of Nallacetone takes place on alkyl chloride of bromide alkyl lodide forms. This reaction is called Finkelstein reaction 

\(C_{2} H_{2} C I \stackrel{N a t}{\longrightarrow} C_{2} H_{5} I+N a C I\) 

Free redical fluorination is highly explosive reaction, so not preferred for the preparation of fluoride.